D.M.
Hi R.,
It sounds to me like he might be getting too much sleep during the day. I know it is very hard to wake up a happily napping baby, but you might try shortening his morning and afternoon naps. For example, if he usually sleeps for more than two hours, wake him up after 90 minutes. And do not let him nap past say 4:00 in the afternoon. That's what we had to do or our son had similar problems to yours.
And when our son got to be about 1 yo, we had to switch to just one nap. Now at 2 yo, we have to be really disciplined about waking him up from his afternoon nap after 1.5 hours. If we let him sleep as long as he wants, he stays up until 11:00 pm or later.
Good luck!
D.