I'd probably go with the Grendel and cut the stock for a shorter LOP with a thin rubber pad. Then after he grows, you can install a 1-1.5" pad to get some of your LOP back. Plenty of good bullet options out there for shots inside 150yds. When my daughter was that age, she shot a youth 700 in 7mm-08 with 120's going about 2,450fps. Both the NBT and the TTSX worked fine. I'd probably go with the 100TTSX in the Grendel, pushed as fast as you can manage. Wouldn't be far off a 250 Savage, which has killed a lot of deer.
