There's a lot of water in most foods too.
Have you a cup rather than a bottle - even a "normal" one either outside or in the bath!

Keep an eye on his nappies - if he starts to have dark smelly wee, or significant difficulty pooing then take more action.
You could also try feeding either: 1) with a definite distraction (if he can see a book or the telly will he feed better?) or 2) with no distractions at all.
good luck