You might want to try an earlier nap. Usually when they transition to 1 nap, they can only manage about 4.5hrs A time before nap & slightly longer after nap... so him taking a long time to fall asleep could be because he is overtired by 12.30pm.
Also I wouldn't go in if he wakes earlier (unless he is crying)... try leaving him there & he might fall back asleep himself.
He's in the transition phase at the moment & it could be that skipped the morning nap for a few days but is still not ready for 1 nap. You might want to try offering him a morning nap at 10am & if he's not asleep by 10.30am then try for 1 nap around 11.30am but if does fall asleep in the morning then wake him up 30mins later & offer an afternoon nap about 3hrs later.
So somethine like this:
7am - wakes
10-10:30 - morning nap
1.30-3pm - afternoon nap
7pm - bedtime (to be asleep by 7.30pm)
He's also doing 12+hrs at night & at this age 13hrs is about the av (nap & nightsleep) so it could be that 1hr is all he needs (or can do). I know its hard to push bedtime if he only slept an hour but thats another thing worth giving a go.... you only really need 11hrs at night & 2hr nap.... so either consider waking him up earlier or push his bedtime about 15-20mins later (& not let him sleep in the morning).
Sorry I hope my rambling makes sence.... let me know if you have any q's
Layla