Quote from Sylvia224:

From what I have heard, this is a very common side effect of Ritalin, second only to heart palpatations. Does he take it with food or shortly before a meal? That might help.
Good luck!

I was on Methylphenidate (the generic of Ritalin?), and although it worked like a charm, after about 4 or 5 months of use, I was hospitalized with pulminary tachycardia and palpitations. It worked so good that I was hesitant to get off of it. When I took it and it kicked in (15-30 minutes later), I felt much better, and easily performed tasks that normally would have taken me MUCH longer due to distractions.

Generally, doctors tell you to take right before meal. Taking on an empty stomache is what usually causes stomache aches.