C.G.
I think it's unlikely to be a milk allergy if your son is on formula. The formula is made of cow's milk as well (unless he is on a special formula) so it probably wouldn't be a sudden reaction. Some of the usual signs of milk allergy are skin rash, wheeziness, congestion, constipation, a red ring around the anus, and gas.
If your son was on breastmilk then it is a different story and could be related. Good luck.