R.M.
Hi Loriibeth-
Has your son been eating and drinking like normal since the fever started? I know it sounds like a silly questions, but if nothing is going in, nothing will go out.
Additionally, water helps with constipation and the pooping process, since your son is ill and feverish, he may be dehydrated and constipated.
Before you call the doc, give your son some liquids and see what happens.
Good luck!
R. Magby