Hi everyone! Thanks for all the suggestions. When he went in for his surgery we did a full blood panel, and had pretty much everything checked. I just talked to my mom and she says they did check his thyroid, as she had the whole 9-yards done. She's going to get some hairball paste today, and will gradually switch his food over. I feed my dog Innova, so we'll see what Innova has at the dog/cat food store in town, for cat food. She's also going to call another vet today. We live in a rural setting, so I'm not always very thrilled with our vets, and sometimes am skeptical as to how much they know, compared to vets in bigger areas. Maybe I'll make some phone calls to some other larger clinics as well!
If anyone else thinks of anything, please let me know!