My dd is the same age and I'm also trying to keep her on 2 naps for as long as I can. I tried 1 nap a couple weeks ago because I thought she was ready, but after 3 days of it, she was a mess of OT. Here's her schedule:
She wakes for the day between 6:30-7am, 1st nap at 10am, 2nd nap at 3pm and bed between 7:30-8pm.
I don't let her sleep past 4:30pm for her 2nd nap, and she usually sleeps 1.5 hours in the AM and an hour in the PM. She's also getting less sleep than your little guy at night. I pushed her 1st nap back by half an hour a couple of weeks ago because she was resisting it so much. Now, she goes down like a charm.
You could try stretching out his awake time between sleep times where you can and put him to bed later if he will do that. Hoping any of this helps...take what applies to you and your lo...I certainly know where you are coming from. The transition for my ds was a piece of cake....my dd seems to be taking her time, but it'll all work out in the end.