nay23- Have you tried keeping him up for the full 2 1/2 hours? By 5-6 mths 2-2 1/2 hours is what the Sleep School I recently attended recommended as the rough time they could be awake and naps should be 1 1/2 -2 hours long....
I found by keeping Iz up for an extra half an hour- she started settling much better and sleeping for longer for at least one nap during the day.