my 11 year old shoots 70 gr 777 and a 200 gr sst It works great and he says it doesn't have much recoil.

If you have the 245 Powerbelt already, why not try that starting with about 60 grains of Triple Seven. You can have him shoot it with the 60 grains, and if he is okay with it, move up to 70 grains. You really don't need more than 70 grains to shoot out to 100 yards, or even a bit further. Even if he wants to stay at 60 grains, that is plenty of powder to take a deer. Heck, back in the olden days they used to use 50 grains of black powder and a roundball to take deer all the time.

70 grains BH209 under 200 grain SST. worked great for me last year, Very little recoil