P.J.
Hi B.,
This was an issue with my daughter as well-- she'd get busy playing and not want to stop to go to the bathroom. Then she'd get constipated and it would hurt for her to go, so she would avoid it even more. Her doctor recommended MiraLAX-- it is an over the counter laxative-- it's kind of expensive, but it really works. When your son gets constipated, just put a little in his juice or whatever he is drinking. It does not taste bad at all-- in fact, it adds a little bit of sweetness to the juice and makes it taste even better. The way the doctor explained it, it helps to regulate their bodies, so eventually you won't need it anymore-- they can tell when they need to go. Her doctor also also talked about how being constipated can be really bad for the kidneys, liver, etc., so you really do want to get a handle on this.
You could also try going a more natural route-- raisins, dried plums, yougurt. Avoid too much bread, pasta, etc. Buy more fiber-rich foods. Good luck!