Then he started getting his 2 year molars, and he got attached to one special cup for milk.
And one day he stopped drinking milk out of anything besides that cup. We bought a spare spout because his teething efforts were wearing out the original. I threw away the original spout in November. We started trying to get him to drink milk out of other cups, but he would not.
The week before Christmas, not long after we opened our Christmas gifts, he broke the replacement spout. He has not drank milk since.
It's a bit inconvenient because when he got fussy before, a cup of milk in his milk cup settled him down like nothing else. On the other hand, it's good because he has not been constipated since he stopped drinking milk.
It is sad, though. He will occasionally ask for milk, then remember and say, "Where's your cup? I break it."