Hey hicham,
SO I understand what mikey is saying about the starving themselves, but this doesn't happen very often, and I would agree with the vet (as my vet said) to take the sunflower seeds completely away.
I'd try getting whatever pellet mix you can (harrison's would be my recommendation). Mix the pellets with some boiling water and let them get mushy like porridge. Let the whole think cool down so that it's just barely warm and offer him this in the morning. Leave only that in the cage with him all day, and then at night see if he's eaten any. If he has great.. give him more... if he hasn't then try mixing in a couple seeds with the mash and leave him that for the night. He'll go for the seeds if he hasn't touched the porridge yet, and get the porridge taste in his mouth while eating the seeds. Try the same thing the next day.
Try this for a couple days and see what happens...
I cold turkey'd my bird, and he didn't eat pellets, but he does now! You just need to watch and see if he is eating at all, otherwise you can't leave him not eating for days on end.
The finer the pellet the better as it becomes mushy easier.
Mikey - I understand your rational for the mixing theory - but birds aren't like dogs or cats in that they will pick through food for what they like... if it's all solid.. they'll only take the seeds and say to hell with the pellets... lol. I work with a very good avian vet, and she s

ays dry mixing is almost impossible ... but thanks for trying to help!