i've always found a few good feed of sultanas get things moving if it is constipation (not so much that they end up with a pain in the stomach though) , also as MG said apple or apple and pear juice are another that helps things along also.

Of course there is the geriatric favourite of prunes but I give sultanas a try first cause they are usually more ameniable to children.

As MG said though it may just be him but if he is showing the other signs of constipation and you are concerned you could see your pharamacist or doctor as I believe there is stuff you can get especially for kids.