It sounds like you have a dry food junky on your hands!
Often, cats that eat dry food become addicted to it and it is very difficult (but not impossible) to get them off it. It took my junky a few months. In my experience, cats simply won't lose weight on dry food. Period.
As for cooked chicken, it's not balanced diet for a cat so I wouldn't give it more than as an occasional treat but that's fine. I would not give tuna to your cat either unless it is only a vary occasional snack. Again, it's not a balanced diet, it's not very good for them and they can become addicted to it very quickly.
If your cat does like tuna, however, that might help you introduce some wet onto his diet but sprinkling some tuna juice over the wet food. Just a thought. Another thing that can help a cat accept wet food is, if you're free feeding, start doing mealtimes instead. Then begin to introduce some wet. Often times, the kitty is hungry enough to eat it then.
As for everyone having a different opinion. I think you will find that, among people who have actually studied feline nutrition (sadly, not enough practicing vets) they DO agree that either a properly balanced raw/cooked diet or canned food are all far superior for a cats health than dry. I know it is confusing when your vet tells you otherwise but, keep in mind, most vets have practically NO nutritional training in vet school and if they do, it's about an hour and a half class given by reps from pet food companies (namely Hill's)! I honestly don't think you could find a reputable vet, who actually studied feline nutrition, that would recommend dry food.