It doesn't sound like too little but you could try offering more and seeing if he takes it. Or perhaps start adding a bit of protein for dinner. What time do you do the last feed of the night and what time is bed time? Do you do a dream feed?
Don't worry about what books say, is you're happy to do the night feeds, and it sounds like bub needs it, keep doing it until it is a problem for you. If and when it does try and slowly wean by giving shorter feeds each time, so bub gets used to having less gradually.