I will assume that the drs checked the usual like appendicitis, hernia blocked intestines etc?
My daughter (15) had suffered for years with severe constipation (others get diarrhoea) & pains about 1-2 hours after eating, this is now resolved as she self-diagnosed herself by searching on-line search engines re her symptoms and discovered "irritable bowel syndrome" which is one of another of auto immune diseases which can mimic each other and which can cause severe stomache pain, loss of (or gain) weight etc etc.
I am not saying that this is your son's trouble merely showing that it can be very worthwhile to do some research on-line via search engines to see what can cause your symptoms & also what can alleviate the symptoms.
Another thing that can cause severe stomache pain which most people & drs forget is allergies!! So consider whether there is anything new around or anything that your son ate or played near on both ocassions - remember also that you can develop allergies to things that you were fine with!!!!!!!!!!