G.M.
Since he can be on whole milk now at his age, and drink from a cup, I wouldn't worry too much about it. You can encourage him to use a cup by saying that, "mommy's milk is all gone now, and since you're a big boy, big boys drink this milk (pointing at your carton of milk in the frig), and big boys drink from big boy cups. I wouldn't even start him on a bottle since he can drink from a sippy cup. Just take it day by day and introduce milk in a cup for him each day until he takes it. Give him other calcium supplemented food like cheese, yogurt etc. :-)