IMHO, if he was doing well before the antibiotics, the last thing you should be doing is changing his food. What works for one IBD dog does not necessarily work for another. If I gave Minnie (IBD X 2 diagnosed 7 1/2 yrs ago) the I/D that SoulsMom uses or the talapia that RobinM has fed, she'd end up with HGE.
If you want to add fiber, other choices include beet pulp, bran buds, or pumpkin. Another option is probiotics, although I've found that what suits one dog may make another worse. I've also had some success with allergy tests, but a lot of vets don't like them, and they probably wouldn't help with the immediate problem in any event.
I hope you can find some answers.