Hi there!
At your lo's age it is common to be on a 4 hr schedule. It is possible for your lo to be both a fast eater and a snacker! I know that the BWSAYP book shows the transition from a 3hr to a 4hr routine but you could just work on getting to the 3 hr routine first. I had a friend who got into the bad habit of feeding every 2 hrs and her little girl put on a lot of weight. When it came time to transition she was able to do it cold turkey (just all of a sudden went from 2hr to 4 hrs). I think that shows that it was a snacking issue. I am not in any way telling you to do it like that!! Just letting you know that you are not the only one in that situation.
Alot of times when babies are ready to move to a 4 hr schedule, their night time and daytime sleep gets messed up. Likewise once babies are adjusted to a 4 hr routine their night time and day time sleep seems to iron itself all out.
I would suggest doing a slow transition to a constant 3 hour routine and then start working on a 4 hour routine. Only start the transition though if you little guy isn't currently going through a growth spurt. If he is going through one then he will need the extra calories. I think the best way to stretch them our a lottle longer between feeds is to use distraction. It always worked for my little guy!
HTH