When it comes to AR15 pistols (PDW's) for self defense use the 300 Blackout rules. It's meant to run out of a 8-9" barrel and with Hornady 110 vmax or Barnes 110 vor-tex it's shows devastating terminal ballistics.
Mine ...
I'd suggest keeping him on the AR15 platform. It's ergonomics and adjustable stock make it a good choice for small shooters. You only need to switch out uppers in differing calibers to suit it to what you're going to hunt. The 6.8SPC, 6.5 Grendel and .300 Blackout are all good hunting calibers...