MY SON IS 12 MONTHS OLD AND HE DOES HAVE LOOSE MOTION AFTER TAKING MILK OR MILK PRODUCTS. PLEASE HELP.?

whenever I try to do some experiment with milk that is when i give him to drink milk or try with biscuit crushed in milk or cornflakes with milk i notice that he would have loose motion after taking that. I want to know whether he is seriously allergic to milk or is there some other reason.