I guess you have to try different options and check what's working.
I would say that problems are coming from UT generated somewhere during day, but agree that in the evening he may be just OT. Have you tried to push his morning A, have a decent nap of 1,5-2h as you have but finished later in the day and than making a short nap after shorter A and again having shorter A till BT?
So ie:
7-11 awake /4h
11-1 nap (if you push the morning A slightly you may get a decent 2h nap)
1-4 awake /3h
4:30-5:00 nap
5:00-7:00/7:30 awake/ 2-2,5h
It would be a great start to 2-1 transition. In coming weeks/months you may push first awake time and first nap, when having second nap between 4:30-5:00 but shorter and shorter and than you will finally cut it out? WDYT?