I'm sorry he is having such a rough time. He sounds underweight so I understand your concern with getting him to eat. Something that I started doing with my j-pouch and still do today is make protein smoothies with lots of fruit and vegetables. It works well as it helps with our fast transit times. I use a protein powder that is all plant based and hypoallergenic. I always have a banana, which helps with hydration, blueberries and carrots. I add another fruit and usually kale in there too. It isn't low calorie. I'm sure they have told you about ensure like products. I never found one that tasted good. My nutritionist suggested the protein shakes. For a while there before my diversion surgery I had 2 shakes a day.
If you add an English muffin with peanut butter it is a good meal that is bag friendly.
I eat more than I could with my j-pouch. I don't eat very many salads because they tend to race through my system without digesting, sorry if tmi.
She took care of my adhesions during surgery so I didn't have a problem of feeling too full when I ate. She spent a lot of time during the removal surgery lysing my adhesions too. I wonder if your son is battling internal scarring too.
I had chronic pouchitis and cuffitis plus the cuffitis was causing continuing scarring at the base of my j-pouch that dilation didn't help. I had dilation during pouch scopes and had a surgical one done that was back immediately. I had to give myself enemas in order to empty my j-pouch. The pouchitis et al didn't go away after I was diverted so I had my j-pouch out.
We all have different problems. I hope the diversion helps clear up your son's problems. I can understand why he wants to see if a revision will work. I wish they would have given him a permanent ileostomy instead of a temp one as the permanent ones are much better to deal with.
He is thin so I hope he isn't having problems with the bag.