You may get a better response in the Traditional forum, but I have loaded my flintlock all the way down to 20 grains of Goex FFG and it still fired everytime and very fun to shoot, just for fun mind you. At 50 grains FFG, the gun is still very accurate and very little kick. How old is your grandson? I love shooting 50 grains FFG all day for fun. I use a patched roundball and PA conicals, but that wieght of Sabot would work well, with hardly any kick. 40 grains should work as well.
Jack