I have read that at this age some babies go to a set nap time. Since you are on 1 nap, which is perfectly great, you could try to move it after lunch- so it is midday- say 1pm?? It might break up that long stretch-and he might be really tired after 4-5 hrs. Maybe the morning nap was too close to wake up time?
If he is a new walker! YAY! He might just be "running" around trying to practice all the time- way to busy for sleep.
That being said- it's great that you're still encouraging the nap. They still need them, so keep trying :wink:
Really sounds like he's right on track for everything. Oh, and if recent naps are falling so late, that is also why he is having trouble going to bed. Sounds really developmental so hang in there!