The first thing you need to do before you switch his diet is to make sure he is healthy enough to withstand the stress of a diet change and a visit to an avian vet can achieve this.  Is his keel bone prominent?  Most birds will lose weight during a diet conversion, so make sure he is of a good weight for his frame before you try to switch his diet.  It is always a good idea to take your new bird to an avian vet for a checkup especially if you have other birds to protect at home.  It is also a good idea to keep your new bird separate from your existing flock.  Not only will this help prevent the spread of illness but it also allows you to see the new bird's droppings and to monitor how much he is eating.

You don't want to starve your bird, of course, and if you see black tarry-looking poops, the bird's digestive system is shutting down and the bird is in starvation mode.  This can be very dangerous.  Also, it can help to have a good gram scale handy because your bird is likely to lose a little weight during this process.  A gram scale will help you keep exact tabs on that.  If you can feel your birds keel bone in front and the bird's chest area is shaped like a "V", then your bird is skinny and you should probably not try converting your bird at this time.

For success in getting your bird to try new things:

Make sure he's hungry when you try.  Remove food cups before putting him to bed, but leave his water dish in there.  It is also a good idea to change cage papers or use a grate on the floor to prevent the bird from foraging for food dropped during the day.  This is only temporarily while you're trying to convert your bird.  Foraging is a good past time for birds and likewise, they'll forage through their food cups if you try mixing pellets with seeds.  This natural instinct will lead them to pull out the seed and ignore the rest even if they normally eat those same pellets when placed in a food cup of just pellets by themselves.

Give them a couple of hours to try the new foods.  If after you've spent time eating and having them watch you, and they still do go for it, go ahead and put the usual food back inside.  This is the same way you'd go about getting your seed eater to try pellets.

Eating is a social thing for birds, so you'll have a better chance on getting him to try some if you're eating (or pretending to eat) some also.  Often it is more fun for them to "steal your food" than to eat what has been purposely put out for them.  That is why putting it in their food cup in their cage and walking away doesn't always work very well.  If your bird has already begun to form a bond with you, that can make things easier as he will trust you when you introduce him to new foods and he will be seeking to fit in with your (human) flock (family), wanting to eat what you're eating, etc...

Contact some of the popular pellet companies and ask them for a free (or low cost) sample.  It sure beats spending $10 on a bag of pellets to find out that your bird doesn't like them.  Cockatiels are generally more veggie-eaters than fruit eaters. Keep that in mind when choosing a pellet for him. Some are “natural” (no dyes) some use dyes.  Some are made based on corn flour, some are extruded (cooked with heat in the factory), others are based on a variety of pulverized seeds laced with vitamins and minerals throughout.

If you had to choose one or two pellets to try (for your cockatiel), I would suggest you try Zupreem Veggie Flavored Cockatiel Diet.  If you had trouble finding that one, I would try Harrison’s Adult Lifetime Fine.  The Harrison’s is made using the bird’s favorite seeds and the vitamins and minerals are mixed throughout the inside of the pellet itself, in every bite.  Many birds take to that one quite easily.  If I had to suggest some pellets that use the dyes (and dyes have been linked to liver disease and kidney disease in studies in which the bird was fed a 100% pelleted diet) – my birds enjoy Kaytee Rainbow Exact Cockatiel and Pretty Bird Daily Select Small.  Kaytee also makes an organic pellet that looks much like the Zupreem Veggie Cockatiel pellet and my birds also ate that one readily even when presented for the first time (but they already enjoyed Zupreem, so they may have been duped, I don’t know). Other ones that my cockatiels enjoy is Roudybush and Zeigler’s bird pellets.

With fresh foods, try presenting them in a variety of different ways.  Carrots are a good source of vitamin A which many parrots lack in their diet, but they're also sweet and most birds do/will like them.  Set yourself up a veggie platter (with ranch dip on the side for the humans only <grin>) and have whole baby carrots, sliced red bell pepper, yellow zucchini squash.  Make it look and sound good to be eating them.  Don't worry about coaxing the bird to eat them at first, just let the bird see you really enjoying them.  Don’t give your bird any of the dip and don’t give the bird anything that has been in your mouth, of course.  If baby carrots don't work for your parrot, then try shredded carrots or matchstick carrots (both can be found in the produce area of major grocery stores or you can make your own).  If that doesn't work, try thawing out some of the frozen carrot circles or the mixed veggies that have the little carrot cubes.  Warm up the veggies so they aren't hot enough to burn the bird. Remember that microwaves can have “hot spots”.  Perhaps your bird would much rather prefer to get those carrots mashed or baked into a birdie bread of sorts?  The National Cockatiel Society's website has a few easy recipes - http://www.cockatiels.org/articles/...RecipeIndex.htm.

Some items my cockatiels love to eat include scrambled egg, warm cooked peas, warm cooked whole wheat pasta, warm cooked brown rice, multigrain toast and bread (dry, no butter), shredded "broccoli slaw" (found in bags, brand names Dole, Mann's), shredded carrots, and dark leafy greens when I shred them like blades of grass using a sharp knife or kitchen shears.  Not only do my babies eat fresh food items with gusto, they also enjoy more than a half dozen different pellets and they have 3 kinds in their cage at all times.

The key to getting your bird to eat a healthier diet is to be more persistent than the bird is resistant and to have lots of patience.  If you become frustrated, your bird will sense that. You have to keep a positive outlook and don't force your bird to do anything as that may break any trust bond you've developed up to this point.  It will take some commitment on your part, but it will be worth it.  Try removing the food cups every night and every morning spend time with your bird and the food items you wish it to try.  Eventually your bird will be hungry enough to try.

My goal is to keep my birds with a daily diet consisting of approximately 50% pellets, 30% fresh foods (usually whatever I’m eating) and 20% seeds/treats, etc… Of those pellets that my birds receive, they receive some that are “natural” and some that use dyes so the actual impact on their liver or kidneys with the dyes is minimal, I would imagine. They seem to leave the red pellets alone, so I put those out for the wild birds. Would you believe it or not but the wild birds here will eat the pellets first before they will eat the wild bird seed that I put out for them regularly. I couldn’t believe they’d go for the pellets before the seed and they are wild, nobody is trying to coax them into healthier eating. The word must be out in the wild avian community that you can feel better and stay stronger longer if you eat those pellets, when you can find them. LOL! Who knows, but that is the truth.

I hope this helps.