what about the norco or kona woman specific bike if you were thinking the tranistion syren.
Let's not forget the ultimate best kids dh specific freeride 24 inch "Kona Stinky 2-4" which is still on the market. I use an old 2003 model and it rocks like the new ones cause I've put it up to date. BTW I'm 15 years old at 5'1 and I shred and destroy on a kona kids bike
At 5 ft he is almost ok for small adult bikes. One of the kids dh bikes would be too small pretty quickly. One option would be to buy a small frame and stick 24" wheels on it till he grows a bit. Stick to air suspension if you want it to work properly.