S.O.
Yes, I think the nap needs to be shortened. 90 minutes, maybe, so that he is more tired earlier in the evening.
And / or try waking him by about 7:30 am and starting his day earlier.
The first few days of the change are going to be icky. Whenever our sleep pattern changes, any of us can get tired and cranky.
Hang in there. After day 5, it should get better. But be consistent.