I don't know if you can really go wrong at this point. Part of me says, just take it away (it doesn't sound like he would be traumatized) and he will drink eventually. The other part says you are doing the best you can, and he isn't hurting anyone or being developmentally delayed, so who cares? I guess it also depends a little on how he uses it. Is it a comfort item that he sucks on (like a pacifier)? Or is it just a habit? I am one of those people who hates to see older kids with bottles, and I don't even believe in pacifiers. But for this, I don't have a clear opinion and am curious what other moms suggest.
Honestly, peer pressure will likely kick in soon and he will realize that this is unusual. I would only worry that he might be embarrassed someday about it. I don't know... but I do have a suggestion. What about sports bottles with the squirt tops? It might make a good transition since you can kind of suck on it, but it is more socially and developmentally appropriate for his age. Good luck!