So as I said, I'm really not one to give advice as we've been struggling with this for 3 months...
It might well be worth trying a bit of an earlier nap in case the longer time to bed helps. Or I think given he's not a crazy early riser, you could push till 7pm for a week and see how that works.
We've played around a lot with timings (mainly unsuccessfully), but where we are now is a nap at 12/12.30 until 2 if I'm lucky, and then a 6.30ish bed or earlier for a crap nap. It still can take up to an hour for DD to settle but sometimes quicker, and I wanted to keep an earlyish bedtime because of this. Later bedtimes it still takes an hour but pushes bedtime too late. And bear in mind DD is up 5.30-6.15am so is an early riser.
Nowadays I'm happy if it takes 30/40 mins for her to settle and irritated but resigned when it's an hour. We have a good BT routine so I just think she needs a bit of downtime right now in her cot to process the day. Her undertired and overtired are v similar but OT does seem to be a bit more active crashing around and crying out rather than just shuffling quietly. She's also a drk room, white noise thumb sucker..
This may all go even more to shit soon though as she is currently settling at nursery and today we will try to nap her. I won't be surprised if we end up with no nap at all (and a 5.30pm bedtime), and at best a 30 minute wonder. The joys...
Life became so much easier when DS eventually dropped his nap and just passes out at 7 from sheer exhaustion.