Maybe try pushing the time he goes down for his nap out a bit? Go 4 hours instead of 3, and see if this helps - maybe he's just not ready for a nap when you're trying it.
I do remember, as some other have said, that DD seemed to drop to one nap at 11months, but then around 13months, went back to 2 sleeps, although she also went through a stage of getting cold after cold at this point, so I put the extra sleep down to them.

Just when you think you have them figured out, they go do something different! lol!