Yeah I would drop to one nap too. It sounds like he's just not tired. Transitions are hard to figure out sometimes, he'll get there. My kids are only 4 and 2 but if there's one thing I've learned about sleep it's this: relax about sleep.
I worried way too much about how much and when and how and by even 4 years old it doesn't really matter. Healthy sleep habits are important, yes. But sleep routines change so so so much in the first few years, most of the time if sleep is approached in a healthy way in the home, it'll work out.