J.B.
the antibiotics could have an effect for sure...
get some yakult or other yogurt or probiotic supplement for him.
also, make sure he's drinking lots of water..
and give him some prunes as a treat...
oatmeal for breakfast...
and maybe ask his doc about a stool SOFTENER till the flora in his guts have re-fleurished... =D
It'll save him from traumatic painful poops.
Good luck =D