What kind of bowel habits does he have? If he is "stopped up" and yet otherwise is very skinny, this could be a BIG reason. I just experienced having a child who lives w/ her mother and gets practically NO fluids, very little, if any...fruits and vegetables and NO exercise. She told me over the weekend she couldn't remember when she pooped and her belly hurt.
I gave her strawberries a couple of times, Mirilax, oranges, water, walked to the park and played with her, etc. From Friday night until Sunday at 2:30, the child (6 years old and about 48 pounds) pooped NINE, YES, I said NINE times. At LEAST 3-4 FEET of feces came out of her over the weekend. She clogged up my toilet FOUR times in one day! I couldn't be mad at her. It wasn't her fault. I was just glad she was getting rid of all the toxins. Her energy level increased EVERY single time she pooped. Imagine that!
She told me she had been to the nurse's office on Thursday because her belly hurt because she knew she hadn't pooped. I could NOT BELIEVE all that was coming out of her. It was so bad that at one point she told me she didn't have to push when she peed. I told her she wasn't supposed to push when she had to pee. She didn't even KNOW that! I realized she was so backed up that her enlarged bowels had been pushing on her urethra as well as God only knows what other organs.
Anyway.......might want to find out.