Do talk to your son's doc or an allergist about this. It is possible to "out grow" a sensitivity but there still may be issues if he eats too much. My daughter was sensitive to lactose, we cut all dairy out till she was 13 months then slow introduced milk (keeping her doc in the loop). She is almost 5 years old and can eat dairy BUT only so much otherwise she is more prone to having painful constipation (even if she has enough fruits & veggies).
Also as a warning to may resurface as he gets older, and could end up being very senisitive or cutting out of their life all together forver. I was senstivie to dairy as a baby (at least that is what they think now, back then it was not really thought of)... I did out grow it and seemed to be able to stomach any dairy UNTIL I was in highschool and the sensitivite started to come back. Now at almost age 30 I have watch very closely my dairy entake, this includes ANYTHING that has dairy in it which a lot of mixed things have some sort of diary in it. In college I became sensitive to eggs, can not eat plain eggs but can eat them if mixed in something... and it is getting worse sadly lemon bars have too many eggs and not enough other stuff so I feel sick after two bites and throat a little tighter.
If you do reintorduce do not over do it, a kid really only needs a certian amount of calcuim (about 500mg for 1-3 year old, 800 for 4-8, 1300 over 9 years). So look at what is needed health wise, yes you need some dairy (or other similar foods) just gague the amount you/little one can handle without getting fussy from discomfort.
An allergy can be more sevear if reintroducing so I would talk with your son's doctor. It really depends on what the reaction was to the milk protein. Violent throw up after eating, breaking out in hives, rashes? Did you get him tested for the allergy? If yes I would have him retested, if no then maybe do get the test done... it may be just a sensitvity not an allergy, you can have both forever and as the body changes/matures/ages the allergy and/or sensitivity can change too getting better OR getting worse sometimes stays the same.