Thank you Tiki. I will wait until tomorrow (its 11.20 pm for me) and hopefully I can read some good advice by then.
If I where to go to a "normal" vet, how can they determine whats wrong? As I wrote, handfeeding cockatiels is really unusual (as well as wing-clipping them

). Therefor maybe someone have a tip for a good medicine I can buy from a vet if someone experienced can determine whats wrong with him. Please tell me how I can help the diagnose (films, pictures etc)
This is a picture of him two days after I got him (7 days old)
He couldnt hold his head up then, the first day he looked like hi still was in the egg (egg-shaped) and his feathers where sticky so I had to clean him so he became "fluffy".
Maybe I should have put him down a long time ago, but I just couldnt

he is such a survivor.