Only guessing here as my DC weaned earlier - before they had wills of their own :), but could you try bribery.When he sees something he likes, tell him that's only for big boys who sleep through the night and drink from cups.
Don't say he can have one once he stops, let him make that decision (otherwise he might revert as soon as he gets the prize) but just casually chat about big children who only drink from cups and glasses etc, so he gets the idea that it's a progress to aspire to.
You could also find some sort of comfort replacement. Tell him that when he feels ready to stop feeding at night, because he's a big boy, then you will get him a big cuddly toy (or similar) and a new sippy cup for night time, and that if he wakes, the cuddly toy will snuggle him and the cup will stop his thirst and he can settle down back to sleep all by himself, like a big boy, but only when he's ready, so to let you know. .
No idea if this will help. It's just a thought. My DS1 was absolutely hooked on dummies. I knew people disapproved, as he was 2 1/2 and still using them all the time. Then one day he tipped them all in the bin, unasked, and announced he was a big boy and didn't need them. I'd been casually mentioning babies who had them and big boys who didn't, but am not sure that made a difference. It was a relief, though, as persuasion had no effect at all.