My DD is 8 months, corrected to 7 months 1wk (3 wks premature). From about 7 months my DD awake time was 2hrs, I gradually increased it by 5mins, and 3 days later I increased it again so her awake time was 2hrs 10 mins. There she stuck for a while, and now all of a sudden it is 2hrs 30mins.
Does he wake happy?
If not, try and increase his nap time in the morning to 1hr30mins, but try and put him down at 8.30am.
What I am saying is increase his awake time for 5-10mins, see how that goes, and if he does well on it, increase it another 5mins after about 3-4 days. He will probably be stuck at his awake time for a couple of weeks, and then see if he can do another 5mins.
Traci (Colesmum) has adviced me, (as I have posted on this board), that transitioning from 3 naps to 2 he should be awake 3+hrs.
HTH