How can i get a cockatiel to gain weight quick?

  I got a white pied cockatiel he's my favorite pet, and i noticed he's been getting skinny he eats, he chirps, and sings, so im assuming he is healthy but i could feel his bones, i give him marvel aid so that could treat his sickness if he does have any, all i want to know and im worry about is for him to gain weight, me taking him to vet is kinda no help for him kuz i have taken a lot of birds to the vet, and they say theyre ok when there not, 3 days later theyre dead!!!! So i was wondering if there is any way to get my cockatiel to gain weight fast???? Or to get him fat???
Try fresh fruits. They have a lot of natural sugars in them, so he should like them. I would try finding an exotic vet and ask their advice as well. I just fed my cockatiels standard feed and they never had a problem. But, I have two large macaws and they get a diet of mostly fresh fruit and pellets. Remember, birds are flight animals, they aren't supposed to have a lot of fat on them.
For one, u do not want to get him fat, birds can have heart attacks from being over weight more easy than other pets, if he seems healthy than i would say hes just getting old. When they get old they start to lose more feathers and tend to lose weight. how old is he?
You don't say what you feed him. A good quality pellet is best, not just some seeds. Does he eat any healthy veggies & fruit? Maybe a little oatmeal would be good. After you take your birds to the vet, does he-vet-know you are giving them marvel aid? Maybe they are getting too much of that. Are they getting clean water every day? After the birds died, were the cages thoroughly cleaned in case they had a contagious disease? Maybe you should try another vet. Did that one specialize in birds? Maybe your bird looks thinner because of his feathers. You don't want him overweight. Maybe your birds are eating or breathing something that is poisonous to them. There are many foods, plants and other things like bleach, paint, sprays, candles, teflon pans, etc. that can kill a bird fast. You really need to check a list on line if you already haven't. They could be dying because you are cooking with a teflon pan or you are feeding them avocados. You should weigh him daily, at least for now, and if there is a noticeable change in his weight, you should then take him to the vet. I know there are a lot of questions here but they are important to know. Hopefully something helps.