I went through this with my son (now almost 8, my 6 yo daughter never seemed to have a problem with it, and my 4mo daughter will hopefully never need a bottle). I honestly forget exactly how old he was at the time though.
What I finally had to do was just get rid of all the bottles in the house. Then there was no "giving in" on my part, and he eventually figured out that the milk was just as good in a cup. :P
With that said though, at that age most babies are/should be still nursing. So perhaps the "need to suck" is still there on his part. Is he still very clingy perhaps? Are all other milestones being reached at an appropriate time? You really have to "read" the child to know when they're ready and only then can you help them break their bottle-addiction.