I agree with PP. Since he's SO new to 1 nap I'm guessing the molars are just enough of a stressor to make 1 nap not doable any more.
If his nap was a bit later in the day (say 12:30 or 1) I'd suggest moving nap 30 min earlier and try that. But wiht a 12 nap, an earlier nap would be 11:15 or 11:30 and *then* if he short naps you've got an awful long afternoon ahead of you or a super duper early bedtime which can mess up your routine something awful.
So I'd probably briefly go back to 2 naps. You may not even have to do it every day, perhaps every other day will be enough to keep the worst of the OTness at bay.
So, I'd do something like this:
7:15 awake
10:45 short nap (15-20 min)
11:00/11:05 awake
2:00 nap (hopefully 1.5-2 hours)
3:30/4 awake
7:30 pm bedtime