Something that I've been trying to tell people with "seed-addict" birds to do, since it is so difficult to get some of them to transition onto pellets, is to simply improve the brand/type of seed-mix you're feeding them. This is something that you can do on the spot, and that is a big enough deal that it will change his health around immediately. And they typically have no issue changing seed-mixes.
With captive/pet birds, the issue with them eating mostly seed is that they aren't flying miles and miles all day long like a wild bird is. However, they are still "designed" to eat seeds, grains, legumes, etc., especially their GI Tracts. So the key is to eliminate the largest sources of fat, sugar, carbs, oils, etc. in their seed-mixes. So instead of trying to make a dramatic change right away from seed-mix to pellets, first change their seed-mix to one that contains NO Sunflower Seeds of any kind, NO Peanuts/Nuts of any kind,
and NO Dried Corn Kernels/Cracked Corn of any kind, as these are the 3 largest sources of empty calories and fat that goes straight to their livers. There are several widely-sold, commercial seed-mixes that are available at any Petco store that are extremely healthy, low-fat, nutrient-dense, and that you don't have to worry about them eating as their regular, daily "staple" diet, and that they should have no issue at all eating right away. They contain healthier, lower-fat seeds, fresh grains, fresh legumes, dried herbs, veggies, and a little fruit. The two of these that I've fed my birds/currently feed them (my birds all get a small portion of healthy seed-mix every day right along with their daily staple of pellets) and that are very highly rated are Higgins Vita Seed "California Blend" and Tropimix. They are both extremely similar to each other in that neither contains any sunflower seeds, nuts of any kind, or dried corn products, and are both full of very healthy grains and seeds (such as Flax Seed, Chia Seed, Hemp Seed, different types of Quinoa, and then a variety of grass seeds, millet, buckwheat, etc.), along with tons of different veggies, herbs, and fruit. Another seed-mix that contains no sunflower seeds, no dried corn, and no nuts, and is full of healthy seeds, grains, legumes, and then some dried herbs, but lacks the veggies and fruit (so it looks like more of a traditional seed-mix) is Higgins Safflower Gold No Sunflower...It's another one that I would highly recommend if your bird doesn't like either of the other two, but that's rare and I've not had any of my guys reject either.
****Something that I'll point-out about the Tropimix seed-mix that gives it an advantage nutritionally, financially, and also makes it much, much more convenient that people don't realize about it is that the Tropimix seed-mix ALSO contains Tropican pellets!!! I switched from Zupreem Natural and started feeding Tropican pellets a little over a year ago on the advice of my CAV. Zupreem pellets in-general are not the best, not the worst either, not by a long-shot, but they contain a lot of "fillers" like cornmeal, soy meal, etc. And my CAV started feeding his own Macaw Tropican pellets a few months prior to him speaking to me about them, on the advice of some of his colleagues who work as Veterinarians/Professors at The University of Pennsylvania Veterinary Medical School; they have started recommending Tropican pellets/Tropimix seed-mix to clients who's birds are refusing to transition onto other high-quality, natural pellets such as Tops or Harrison's...And my guys all LOVED the Tropican pellets straight away, I had no issues at all with all 4 of them eating all of their pellets right away, which they wouldn't even do with the Zupreem Fruit pellets that are full of added sugar...So we made the switch to the Tropican pellets, which are a bit cheaper than the Zupreem as well, so that was nice too...
At the time I switched them onto the Tropican pellets I was feeding them each 1/4 cup of Higgins California Blend seed-mix every day as well, which they loved and I had no complaints about at all; ironically the Higgins California Blend is extremely close in recipe to the Tropimix, they look almost the same too, which is a big deal to a lot of birds. So I just stayed with the Higgins California Blend seed-mix and switched to the Tropican pellets...Then at an appointment for my Cockatiel with my CAV (she had broken a blood-feather and I had to yank it out with pliers, it was awful), my CAV asked me if I was "only buying the Tropimix seed-mix now", and commented on how awesome it is that you only have to buy an all-in-one food and not worry about buying both pellets and a seed-mix anymore...I had no idea what the hell he was talking about, lol! I was like "No, I'm still feeding the Higgins California Blend seed-mix, but I did switch to the Tropican pellets and they love them, but since the Tropimix and the Higgins seed-mixes are basically the same thing, I just stayed with the Higgins"...Then he apologized for not filling me in on the fact that the Tropimix seed-mix also contains plenty of the Tropican pellets (each cup of Tropimix seed-mix contains about 1/4 cup of Tropican pellets, so it has a lot of the pellets in it). So now I ONLY buy Tropimix seed-mix, and instead of feeding them a 1/4 cup of seed-mix and in a separate bowl giving them 1/4 cup of the Tropican pellets, I just give them each 1/2 cup of Tropimix seed-mix and they get both their pellets and seed-mix in one bowl. And they usually finish their entire bowls every day! (I feed them their pellet/seed-mix in the morning so they have it throughout the day, and then at dinner time they get their fresh veggies/greens, usually in a "chop" mix, and they eat it with me while I'm eating my dinner). And the Tropican pellets are naturally sweetened with fruit juice, which is still sugar, but it contains half the sugar of any of the "Fruit" flavored pellets, such as Zupreem Fruit. So I'm fine with that...and the love them, so I'm thrilled to death. I only have to buy one type of food for them now, and the Tropimix seed-mix is pretty affordable, both in the Petco store and even better online...I order mine from Chewy.com (no matter what seed-mix/pellets you choose, Chewy.com is typically the cheapest place hands-down)...The only thing is that I have 4 birds of different sizes, so I have to order 3 different "sizes" of Tropimix, but if you only have one bird then it would be amazingly easy and cheap. They sell the smaller 4 pound bags of Tropimix in the Petco stores for $9.99 and they are $6.99 on Chewy.com with free-shipping, but Chewy.com also sells a 20 or 25 pound bag of Tropimix for around $35-$40, depending on what size you buy, so that lasts forever if you put it in bags or containers and keep it in the freezer.
Something to think about...Again, simply switching his seed-mix to one that is totally free of any sunflower seeds, nuts of any kind, and dried corn of any kind will make all the difference in the world, and is something that you can do immediately...Then after he's on a healthier, low-fat seed-mix, you can start transitioning him over to a natural pellet staple gradually, at his pace.