Some LOs are like that - can handle almost a full A after any length nap, particularly earlier in the day. Often though the tiredness will still accumulate so that they can't last as long after a short nap later in the day as they can after a short nap earlier on. I'd go for BT maybe 1h45ish after your CN - WDYT?
The other possibility is that his 'true' A time is higher than you think, but he's been chronically OT so done some good naps on 2h10 A to catch up. The way you would know if that's the case is that his naps will start to get shorter again after 2h10 A, typically 45 mins - 1h20 and he'll be waking happy. If that happens, time to increase his A time a bit more
There can be some MASSIVE jumps as you get closer to 6 months.