I have a nether land dwarf to that's one years old he did not drink out of his bottle that was set up in his cage so I put a bowl in his cage and filled it with water so he has a choice but if u do get a bowl make Shure he can tip it and it should be heavey becuse my netherland dwarf likes to throw stuff around
If the rabbit has been eating lots of veggies, then it might be that he is getting enough water from them. If the water is in a bottle, change it and put some in a bowl. You can also try leaving fruit juice in the bowl and see if he drinks it. Talk to a vet if the situation doesn't improve.