Hello,
I had the same issues with my son at that age when we were trying to transition to one nap. He was also teething (molars), and waking early. I pretty much went cold turkey with the one nap. I found that when he stared to get tired at his usual am nap time, I would give him a snack and this would give him some energy to make it a bit longer. I then picked a nap time that was in between the 2 nap times, so around here it was around 11:30. Some days early on he only slept for 45 min. as I am sure he was overtired, but he quickly settled into it and then was napping at least 2 hours at a time, and still does.
Hope this helps!