Karen, I think you're right about not making Magic's immune system have to do anything more at the moment. Perhaps don't worry about dieting, either, until you've seen the vet and have a better idea of his status.
His condition sounds rather like my Lucy's was (FIV positive). I had her for four years, she was probably 8 or more when I got her, and her only visible symptom of FIV during all that time was stomatitis, which Magic obviously has. We have had much discussion of this on the list recently, so you can look in archives. Basically, for Lucy dental prophys worked at first, along with antibiotics (antirobe and clavamox, both used for soft tissue infections), CoEQ10, Vit. C and interferon. Ultimately the only thing that helped was full-mouth extraction. It's a difficult choice - in Lucy's case her mouth stank and you could smell it from yards away and she couldn't eat (but she was very hefty and never wasted away), so we had to do it, but I feel that the stress of anesthesia probably is what precipitated her slow downward spiral, starting about 3 months after her extractions were done. For a while, she was able to eat very, very well and had no more mouth odor or stomatitis, but a canine which we left in broke off, and the jaw-bone became infected. This may have begun to happen even before the extractions were done. Taking away all the teeth minimizes or eliminates the bacterial infection that can cause even more serious problems, like osteomyelitis of the jawbone. One thing we never did try but that is supposed to help stomatitis of this type, in particular, is bovine lactoferrin, so you might try that, although it isn't cheap. I think if it were me, if his bloodwork and checkup show him to otherwise be in good health, I'd probably get his teeth extracted, as you will only have continual problems otherwise.
