Hi S.,
It sounds like he is telling you he's ready for a real cup.
We got a couple of unbreakable kids cups from Target, though our daughter (same age as your son) wants to drink from whatever kind of cup we're drinking, so I generally give myself a plastic cup, too, and let her drink my water alternating with her milk.
You can talk to your own pediatrician, but ours said that our daughter should be transitioning to cups, and not to be worried that she will be drinking a lot less milk than she did when she was primarily using a bottle.
I think she said to expect 8-16 oz a day?
We have some sippy cups, but she's not really into them. She's either drinking from a bottle (or breast) or a regular cup. We're going to gradually offer the bottle less and less.
Good luck, it sounds like you're a wonderful mother!