What can I do for my cat's dry skin?

He's 6 years old, Siamese. He's my whole life, my best friend actually. He has a few fleas, and I ordered the pill Capstar for him, which he's had before from the vet and it killed all of his fleas. That's been a couple years ago and he's since gotten more on him. I brush him and sometimes I use a flea comb on him and I can get a few fleas off him from that. But when I pet him and rub his back, he goes crazy and starts licking the air and everything in sight. His skin flakes and he has "dandruff" so to speak, and also flea poop in his coat. :/ I don't have the money right now to take him to the vet but the capstar will work for the fleas because it has before. I feed him Purina, and I just learned that Purina isn't so great afterall. He likes the taste and is sometimes picky about his food since that's what he's always been fed. I did the True Blue survey online and found that Purina had all that corn gluten/animal byproduct, etc. And I'm not okay with that cuz I want him healthy, he's a little overweight since he's neutered..I know that's no excuse, but whatever. He's also declawed and he does not go outside because I have a little dog that likes to chase him and we don't have a fenced in yard and I'm surrounded by woods, so that's a big no go. Anyway, I got coupons to try a bag of Blue Buffalo cat food since they don't have all that artificial stuff, and I feed him wet canned food sometimes, but I try not to do it everyday because it gives him diarrhea sometimes and he also cries for it. All I'm wondering is if there's anything I can put on his coat for the dry skin and itching around the house or something not too expensive that I can buy? I know switching his food will help and feed him wet food for oils and stuff, but if there's anything I can put on his coat to help him get some relief from itching, that'd be great.