Nedra is on Hill's I/D Allergin Free now, as this is the ONLY food that she can tolerate. We've burned through 7 different protein sources that she now cannot tolerate and this is the only thing that we've found has worked for her.
My take? If it ain't broke, don't "fix" it. It sounds like he's doing great on the Hill's food, and because of your pup's outstanding health issues, I would be hesitant to change anything or add anything to his food. Sounds like he's finally gaining weight for you and maintaining a solid stool which is great. I know that you've walked a long road with him and I'd hate to see you have to take a step back where his health is concerned.
Satin balls are about the last thing that I would try for a dog with Colitis and/or IBD. Colitis, IBD and food intolerance is a protein problem. Loading up on proteins with a dog that has protein sensitivities is not a good idea.
Granted, I strongly recommend *against* Hill's products for generally healthy dogs, but I also feel that you have run the gamut with your sick dog and that you should now continue on with what is working for him.
I agree with the others that he's still a pup. That he is gaining is a good thing. Just stay your course and keep at it. About the only thing that I would consider is to add a digestive enzyme and probiotic supplement to his food. I give Nedra a heaping teaspoon of Nature's Farmacy DogZymes Digestive Enhancer in powder form at each meal (
http://www.naturesfarmacy.com/DOGzyme.htm ). It takes some of the stress off of her Pancreas and promotes the growth of good bacterias that aid in digestion in the colon. I also add a quarter tsp to Mia's food, as I believe in supplementing this in times of stress, growth and during courses of antibiotics.
Good luck! It sounds like you're on the right track!