Hello
Pizza , burger, spicy , oily , greasy  , excessive coffee, tea ,chocolates  & overeating are the predisposing factors which causes hyper acidity or gastritis or nausea & vomiting ,hence 
stomach upset . 
Try raising the head of your son about 4 inches with block. It might also help to avoid eating or drinking for 2 hours before he lie down. 
Also give him some antacid syrup but if symptoms are sever he may need some 
proton pump inhibitors like pantaloc- D once or twice in day .
Now for 
loose motion or diarrhea ( but you mentioned that he is having semi solid stool ) ,this is only due to 
indigestion and tension. 
But sometimes amoebic colitis may be the reason of such stool so give him "
SECNIDAZOLE FORTE " two tablets stat with pantaloc-D in a single dose after meal .
Lacticx acid bacillus also help in semi solid stool along curd.
Hope this information will help you.