Is Prolonged Diarrhea In A Child Having Cerebral Palsy A Cause For Concern?

My 6 year old son has Cerebral Palsy and has had diarrhea since 6 yesterday afternoon. I though it was normal tummy troubles that we have quite often. It is 26 hours later and he still has it. As of an hour ago, it has became yellow, has a foul odor, burns, and is foamy. Is this a stomach bug or could it possibly be more serious?