Probably just needs his stomach to mend if he threw up that much. Offer him just little sips of water until he is able to keep them down. Most likely if he threw up just water he drank too much at one time.

Little sips every 20 minutes or so and then when he can keep that down, a little more. Maybe keep him quiet and away from the rest of the pets. I know when Casey would throw up she wouldn't want to come back into the house because she thought she was bad. I never figured out why she thought throwing up was bad because we never scolded her for it.

Hope he feels better soon.