H.L.
C. - you didn't say whether he takes 2 or 3 naps. It's probably time to start the 2 nap a day plan. with the first nap starting 2 hours after waking(likely 9). Then, wake him from first nap after no more than 2 hours (11-ish). Put him down at 2 for the second nap (waking no later than 4). If he wakes during a nap and cries, he needs to know how to put himself back down. Does he know how to put himself back to sleep at night or do you need to put him back down? It may be time for some sleep training to teach him to calm himself back down if not. I found from that age on, it was better to not go in. Because there's no light cue (darkness at night) to tell them it's time to get up, your even entrance to the room says it's ok to wake up. Does that make any sense? If that deosn't work, it may make sense to try a 3 nap schedule instead (even if it's going backwards) until he can stay asleep longer).
If you haven't read it, I can't say enough about Weissbluth's book.
Other random thoughts - is he putting himself down or are you rocking him down. And do you have a "nap routine" either a song, prayer, "goodnight bear" stuff, etc. to give him the cues that it's time to wind down and go to sleep?
He may also be teething. Have you tried some tylenol at nap time? I don't think the anbesol lasts more than a minute.
So sorry. May just be phase, but don't be afraid to knock the routine around a little. I know it's scary to mess with when you have a such a good night sleeper! Good luck!
(I think I know you C. P. If so, call me to discuss. It SUX - just when you think you've got it all figured out..)