We never had any success with fad diets. Beyond switching to LF milk. We tried pH diet ( she was super healthy at this time) but then she had a huge flare... We also tried GF and honestly the only way I could get her to stick to it was to do it myself. GF was a really constipating diet for both of us, so rather than doing GF I would recommend more of a whole foods diet. Both me and my DD were chronically constipated from GF, so it depend on the individual. The GF prepackage processed foods aren't any better for you really. But if you have an itch and need to scratch it you can buy them. Personally most of it tasted like sawdust to me. I guess in time you can get used to sawdust. They are improving because of celiac being so dominant now. I do have a decent list of yummy GF recipes but they are few and very far between. Most of it has to be homemade for it to taste good. But if he suffers from diarrhea GF might be great for him, to bulk him up a bit. We now really just try to avoid GMOs, preservatives and pesticides. So we eat mostly organic if we can. Is it just milk you have cut out or all dairy. Bc organic yogurt is soo good with the probiotics, potassium, Vit D, calcium... It is a win win in my book. My kid is Lactose Free and yogurt doesn't bother her because it is heated. But to each their own.
Beyond that there is the Paleo Diet, Makers Diet and SCD. Some people it works for others it does nothing.
EEN doesn't really work for UC it is more a CD thing. (Farmwife) but I would highly recommend getting nutritional shakes. We used Nutren Jr. So you would need the milk free equivalent. She loved them and gets excited when I give her them. It took a lot of work to get her to that point though. But if you tried EEN and it worked it might be another piece to the diagnosis of which IBD your dealing with.
Personally I was never able to find a trigger. Food in my opinion didn't make any difference with my DD, she like your son never showed any signs of distress from foods, or ever showed an intolerance to pain. It is natural for us to want to control it. Some kids diet makes no difference, others it does. A western medicine nutritionalist will tell you food doesn't matter with IBD, if it bothers them don't give it to them. If you want to get real advice talk to someone in holistics about incorporating anti-inflammatory food into his diet, like sweet potatoes, avocados, salmon. I think adding foods rather than taking them away was a huge factor. Fattening up on the good and skipping the rest. It took me a year to figure that out. A very stressful money wasting year.