Babies do often drop their milk intake at around that age. From what you describe he doesn't seem thirsty, so he must be getting what he needs. A baby is not having enough to drink will have dark, smelly wee and not much of it, and will most likely be very unsettled.

How are you offering the water? If you are using a sippy cup, perhaps a straw cup might be more successful? It doesn't matter if he won't drink it now, but it would be good for him to be able to drink it when he does need it.