Current position is too short.
Drops position is ok but tops position way too short.
I'd go with the larger frame since it's longer. You can start with a shorter stem and go longer as he grows. If you start with a 90mm stem you can add to the position reach up to 50mm. So the end result would be a 140mm stem. And if he outgrows that you can again get a new frame.
Maxing out seatposts is rare these days since posts come in 350mm to 400mm. Of course more than two fistfulls of post showing will possibly make the riding position a bit low so don't cut the steerer tube in the next bike to a minimum. Leave a bit room for spacers under the stem.
So a long post, long steerer and lots of stems should make the frame last a bit longer. Growing children are a headache in clothes and bikes.