Iv always used bottles, and my buns have always varied with how much they drink. on a typical day one of my buns could drink 200-300ml, while another bun about the same size drank 100-150ml.
I have another bun who was used to a bowl, but i always leave a bottle with her too, and she sometimes drinks from it, and seems to drink about the same from the bottle as she does the bowl.
My current older bun doesnt use a bowl- i did try switching over to bowls after someone suggested it may be better to leave my bowl drinking bun with a bowl rather than swap her over, but she refused.
My male who was kept with my female who uses the bowl started using the bowl by copying her, but after a few days went back to his bottle even thoguh there was a bowl there.
I personally use bottles as i feel they are much more hygenic and can hold more (the bunny with the bowl gets bedding, lots of fur, the ocassional pellet, iv seen poo once or twice in her bowl, its not very nice! And shes tood in it, knocked it over once...)
So i wouldnt worry about your bun drinking that much, as Daisylop said, aslong as poos and wees are fine, and he seems to drink from it ok with no struggle, then i personally wouldnt swap over to a bowl either (but it is upto you, it may be worth seeing if he prefers the bowl, and if not keep the bottle)