Could Diesel have EPI? Someone posted this link the other day when my puppy was having issues:
EPI in Dogs
Also, if it turns out to be IBD, he'll probably be fine. It's just a matter of figuring out what food his tummy will tolerate. IBD is similar to Crohn's Disease in humans. I actually have Crohn's, and did a lot of research on IBD in dogs when Violet was sick.
I hope he feels better soon!