Yes I guess it's the whole day routine so a mix of a long day but also a long nap late during day... I have a friend who did have the same issue and almost nothing helped.
The case there was a 2h nap as at your case, quite late during day and overall sleep of 2.5h per day. Together with nights her son was doing 13-13.5h of sleep. She tried various BTs but he was just doing 10,5h nights no matter what BT she offered. Finally she accepted quite late BT of 8pm, he was waking around 6:00/6:30, she was then doing a short 15min catnap and 2h nap around 1:30/2:00pm. He slept 2h always on this second nap.
In your case I would probably:
- think of capping morning nap to 15min (maybe it will help) OR
- think of capping afternoon nap to 1.5h & bringing BT earlier to maybe 7pm
- remodel the whole routine to long am/short pm but long am nap should probably be pushed to at least 11am
- accept it until he is ready for 1 nap (my friend is currently doing 7:30/6:30 night and around 2h nap but if the nap is shorter like 1h, she goes for earlier BT and her son started to catch up).
HTH