M.W.
Just keep offering him the cup. My son did the same thing when I quit bf'ing at 18 months. He didn't drink much milk at all for about 6 months. Just a sip here and there. IT"S OKAY! Just make sure to offer him a well-balanced diet and this won't be an issue.
And be sure to offer him a cup of water throughout the day as well. We always had a fresh sippy of water sitting around so the kids could take a drink whenever.