If I were you, I'd switch to the sippy now since he is still not interested in the bottle, but if you want to use sippy cups for all other drinks and the bottle for his night bottle, that's fine as well, but most pediatricians do recommend ditching the bottle after 12 months anyway...not that it's a huge rush, but you may as well. Also, if he never drinks milk again, he'll live. So, if a sippy of water before bed works as a substitute for his winddown routine, then so be it. You can keep offering milk whenever you want, but if he is eating cheese and yogurt and getting calcium and protein from other sources, he won't be nutritionally deficient if he refuses to take in milk. I wouldn't flavor it either, just becomes another habit you have to try to break later. You can try different cups or a regular cup with you helping or with a straw. Might interest him a bit.