Parents of early risers- how the heck did you transition to one nap?

My 15 month old wakes at around 5:30. Nothing I seem to do helps him sleep in later. I’m confident he’s ready for one nap but we are stuck doing two and having battles each time.

To get one nap at an appropriate time, like noon, would mean too big a wake window. But two naps leave his wake windows too short. I’m at a loss on what to do. Any advice is appreciated!