Was your son involved in all the upgrading? My experience is that kids generally go for new-and-shiny over old-and-worn, but that can change when the kid is the one bolting together the parts.

Have you asked your son which he prefers? Maybe just ask him.

cbrstar mentioned fit. Here is a link to a sizing chart with some general guidance:

SIZING FOR BMX BIKES - Find the right size Eastern Bike

And USA BMX also has a chart:

https://www.usabmx.com/tracks/1709/pages/492