It is hard to say if your rabbit is drinking enough without knowing the size of the bottle that he has drunk a quarter of and the size/weight of the rabbit.
Some rabbits prefer to drink from a bowl and it is certainly easier for them when they are poorly. If its hot and ours are in their runs I give them a bowl as well as their bottles and they always tuck in.
Also if your rabbits eat a fair amount of veg and or grass you may find they drink less as they get more moisture from this sort of food.
If your rabbit has had diarrhea I would be very careful as they can get dehydrated and go downhill very quickly.
Did you give a new veg or food which may have upset him?
If you didn't and he does have diarrhea and/or is not himself, or off his food I would certainly get him to a vet asap.