Try having him lie on his back while you "pedal" his legs. Sometimes the movement helps get things going. At the least it should help him feel better.
Ask your doc about Miralax. It's not a quick fix, but more of a long-term solution to constipation. Your son might start withholding because of the pain and discomfort he's in. I'd want him on Miralax for a few weeks to get him back to normal.
Good luck! We dealt with years of constipation with our oldest.
Tara
living a crazy life with 3 boys
I am thinking now
of grief, and of getting past it;
I feel my boots
trying to leave the ground,
I feel my heart
pumping hard. I want
to think again of dangerous and noble things.
I want to be light and frolicsome.
I want to be improbable beautiful and afraid of nothing,
as though I had wings.
~Mary Oliver