It's possible that he's just holding it in. I say this as a child that was constipated until I was about 10 or so. It was a weird psychological thing where I didn't like the feeling of pushing, and when I did go it was much worse. Overnight I decided that this wasn't ok, and I started going when I needed to instead of avoiding it.

My mother tried all the tricks, but I wasn't really constipated due to lack of nutrients, it was just me. Maybe have a talk with him to find out if he's just uncomfortable with going.