Yes, I think over a period of time the body can build up a resistance to meds to where they may need more for them to work. But in some cases, like steroids, they can actually be reduced to achieve the desired effect. It's just hard to say.
I'm glad he's going to the Vet today though, as not eating is never a good sign. Is he on appetite stimulants? We found with our Callie that she vomited on Mirtazapine pills, but she tolerates the transdermal gel very well, with good results.