J.L.
Well I have the same problem with my 3yo, he had tons of food aversions and medical issues, but what worked for us was getting cups with the flexible straws and "bribing" him to drink. We pause a movie he's watching or he takes a sip or two for each piggy back ride around the house. Its a lot of work, but you do what you have to do. You could just try different flavors in the milk, like chocolate or strawberry and see if that makes a difference, Pediasure is very sweet so you could mix that in with the milk for taste also.