Maybe this is his normal drinking pattern, or it may be due to his diet. If it is high in sodium, salts, it will dehydrate more, so will drink more. If he seems fine, don't worry
Rabbits contain a lot of water it means they are healthy if they don't drink a lot it could mean they are depressed
Measure the mount of water being drunk, if it is over 100ml per kilo of body weight per day then call your vet and ask for advice. It could be kidney disease?
Eating lots of hay will make a rabbit drink more, however, a change in water intake might mean kidney disease. Call your bunny's vet.
Could have a bladder infection