Hi
If the nap is occuring in the morning, then that is the problem. Early wakeups are often reinforced by early morning naps. Baby keeps waking up early because he knows that he'll be allowed to fall back asleep in a few hours anyway. (We got over this a while ago) Our lo is only 12 1/2 months and he is only on one nap a day too and he's just fine. Yes, this normally does occur later though. What I would suggest is that you slowly push back his morning nap by 15 minutes or so everyday until he is napping after lunch. When we did this we had three weeks of crabbiness, overtired baby and mom, and 45 minute naps. ....it does get better. If you just stick to a later nap (in increments so you don't shock the poor guy of course, he should adjust, just be consistent.)
Our sleep scehdule now is 7 - 7 (sometimes 6:45 am) with an afternoon nap from 12:30 to 2:30. If we did it, anyone can!
Good luck
I'd love to hear what happens
Zelka