Dropping to one nap

I have a (nearly) 17m old who is on two naps. He sleeps through the night however hes fighting his last nap and is only having a 25-30 min last one from 3:25-4pm. I don't wanna push it past 4 pm because then his bedtime gets later so 4:10 pm is the max that I will end his second nap. He sleeps at 8 pm or 8:15 pm and wakes up between 730 and 745. I just really don't know when he'll be ready to drop to one nap and how it works.

His wake windows:
3.5/3.5/4

I know his wake windows are short, but that's only because he sleeps at 8 pm and I don't want to push that any later. It's just hard because he is on two naps and I think one longer nap would be better, but unsure if he can handle that wake window if hes sleeping at 8.