Well, he's on 3 hrs like clockwork, isn't he? Sleeps well, otherwise?
I know he's a big baby, but mine was 8 lb 11 oz at birth, and it was 3 months before she slept a 5 hr stretch. I know they "should" give you that stretch, but sometimes they don't, and you may just have to ride this out until the switch kicks in. BUT...you need to sleep too.
One thing to try is instead of spacing the afternoon feedings in 3 hr blocks, try to "tank him up" by feeding at 4, 5:30, and 7:00, and then doing the dream feed at 10:30. Try that for a week, and see if it helps.