You have the interest and ability of this sport clearly shown in you son. I can understand your concern for safety. IMO I would not let him alone when he loads his rds. Double loads are too dangerous to allow him to "solo" period especially at 12 yrs. I'm 68 and as my wife says; I act like a 2 yr old and do dumb things. Point... my brother is 60, weak left leg and arm, fishing with me at Lake Fork, Tx, jumps off bow of boat on ramp and breaks bone in heal and still limps after 6 mths. Former Marine and said "easy jump, hooah" -- he was not thinking. I, he and WE all act like kids so what will a 12 yr old do? I would let him "solo" all but 1 and insist that you check each powder round before he seats
Don't really understand the rational of the expense factor when one looks at the results of 1 pound of gun powder. Consider the number of rounds you reloaded from that 1 pound bottle of powder which I get and avg of 1750 rounds per pound of Bullseye. Can be alot AT the out set. Now consider the # of rounds you make and shoot WITH THAT SON. Maybe 50 each session? Thats 250 times you are with your son shooting for a couple of hours. Wow. I'm 68 yrs old and would have loved to have my son with me in one of my hobbies. Enjoy the time as when your my age, you'll remember those days when you're alone and he is making his own life.
Sorry for the ramble but I've learned the hard way that money is at the bottom of my list when it's "happy time" with my family.