MyTeen,
I think you'll find that researching the diet/nutrition approach to treatment, is more confusing than medications. Unless flaring, most GI's will say to eat anything. I think it's wise to stay away from nuts, seeds, corn, popcorn, hard fibre. Those are typical avoidance foods.
The problem with some of the diets are, that teens are typically fussy eaters. Most of the diets aren't the most appetizing. This is just my opinion right now. I may change my view on that, so I encourage you to keep trying to gain as much knowledge as possible.
Just back to medication for a second. I forgot to mention something that Dingleeire pointed out. Taking 6mp has to reach a theraputic level to be effective. Your son would need a couple of metabolite blood tests, to see if the medication is at the optimum dose. It's different for everyone.
Once again, Good Luck and stay positive.