Erm. Does a 21 month old jumping on and off his bed while laughing hysterically count?

To me it sounds like he's going pretty great to be able to self settle so well at his age. If he's happy to babble away to himself before he falls asleep and no screaming then that's great, I'd just leave him to it and only tend to him if he's getting upset. Some kids have different ways of soothing themselves, you know? DS didn't start to self settle until he was 17 months old (and that was only because my milk dried up as I was 16 weeks pregnant and he had no other choice!) so you're doing a great job!
And I agree, it does sound like he may be heading toward only needing 2 day sleeps. Maybe dropping one may make him that tiny bit more tired and he'll nod off quick. And please, try not to get stressed over it!