RE: pellets or powder
Welcome to the forum. As a general rule, if I want to shoot pellets then I use a 209 primer or one of the other adaptations of cartridge ignitions. Musket caps and #11 caps are best served in the loose powder market. I have a black mountain magnum and it is supposed to shoot pellets with musket caps. But I would not be upset when it failed. I would just use loose powder.
If your son wants to shoot pellets with his musket caps, have him dump 10 grains of Black Powder down the barrel first, then load the pellets. By duplex powderingthe load, the musket cap will flash that back powder and in turn ignite the pellets.
But actually have him switch to loose powder and all his problems will disappear. What kind of rifles are you shooting?