YOu should call the childs doctor , dont rely on the internet when it comes to your childs health.
my son gets like that occasionally here. he's' 5. but usually at that point he is a little constiapted. It sounds bad, but it works fast and is easy.
Try giving him a Fleet glycerin suppository child size.
See if that helps, dropp me line or note here.
Talk too you soon buddy,
Matt