Hi -- pred can just plain do WEIRD things. It can cause some really nasty muscle spasming which can be exhausting and you might not even "see" it happening. He can just plain feel horrible, bottom line.
Something that worked absolutely superb for us when we had a dog with mast cell (same type of grade) where we *knew* we didn't get clear margins (because there was no more tissue in the area to take) -- I used a *second* vet who did "TCVM" (traditional Chinese veterinary medicine).
Muffin got acupuncture and some Chinese herbals and we found out later that we actually hadn't gotten the primary tumor at all but what we did with the TCVM battled that unbelievably well.
I'm not suggesting you leave your vet -- not at all. But rather simply *add* the TCVM to it. No bad side effects and frankly way cheaper than something like chemo. Mast cell usually do multiply if unchecked. But the side effects of pred can be horrific. http://www.tcvm.com is the Chi Institute and on the left they have a 'locator' that will help you find a competent vet near you. They aren't going to try to talk you into leaving your vet or anything like that. It really does help enormously.