OP here, yes he's transitioning to one nap a day and it's been brutal. He wakes around 630 and by 930 is an absolute mess crying to go to sleep. He used to go back to bed at 830, so I've had a margin of success pushing it back as far as it is so far. MB would like for both kids to be on a 12/1230-3/330 nap schedule. Her oldest DC goes to preschool and that's his schedule. She'd like baby on the same one so they can get out of the house on the weekends, etc.
For a couple of weeks I was letting him have a short morning nap (less than an hour) but that totally backfired as then he wouldn't sleep at all the rest of the day. He's also a new walker and cutting 2 bottom teeth ATM. Poor thing is just a hot ball of a mess
Anyway, I think you're probably right in that he's overtired when he lays down and that triggers these terrors. Back to the drawing board to get these naps sorted out.