On the other haned constipation may be caused by something other than adhesions. My mother says I was born constipated and I don't think I was born with adhesions. When I was about your sons age, I was staining so much to have a bm that I caused a fissure in my rectum. Adhesions did start forming pre-teen for me though due to endometriosis, followed by more digestive problems.
I don't know if this helps or not. If your son does have adhesions, and the doc wants to up his fiber intake, you may see some problems (ie., sevre bloating and even more clogging up, discomfort). Many of us on the board who suffer from constipation due to adhesions take an herbal laxative called senna, though I think this would be pretty tough on your child's stomach.
Have you thought of taking your son to an acupucturist? That is what I would likely do in your situation.
Hugs,
Janet