I think that the fish is the culprit. I want to inroduce a protein after maybe another week with his improvement on the veggie diet, maybe beef. What do you think about cheese as a protein? Or is the dairy a bad idea? I was told before that yogurt may help.
I really don't like beef as a protein in general... this is because it seemed that corn and beef were the most common problems that I saw over the years. Of course, some may not be allergic at all to beef, but I always stear clear of it. Venison, Rabbit are good choices for you, it sounds like -- IVD makes excellent diets of both. There is also kangaroo out there which is supposed to be great, but I don't know the source .. and it kind of creeped me out to think about eating kangaroo!. Lamb and Beef tend to cross react some, so avoid both.

Yogurt can always be very helpful to the gi tract as a source of probiotic / helpful bacteria. Careful if you are thinking about "probiotics" as some sources are beef flavored powders (you have to think about EVERYTHING!).

Snacks: I used to have clients buy the canned version of whatever diet they were feeding and then make bite sized cookies by baking in the oven at about 350 degrees, for 10-30 minutes (experiment).

As for cheese, temporary, occasional treat, maybe, but not a good a good enough source of protein for long term.

Sounds like everyone has given you some great advice. Good luck! Perhaps when the summer / fall pollens are gone he will be more comfortable... it only adds to the problem sometimes.

Jes