Forget trying to rationalize this with talk about liability.
Just tell him the truth. You're happy to help him learn to reload, but you're not willing to let him (or anyone else) use your equipment or workspace unattended, or even attended on anything approaching a regular basis and that you're not going to let him store any of his equipment at your house.
That's not at all at all unreasonable, and he should understand that.
By the way, saying: "No." is a lot easier if you just say: "No." People are tempted to provide reasons but that just complicates the matter and gives the opportunity to attack the reasons. Just saying: "No." without providing reasons keeps things very simple and very easy.
<<<Edit. I didn't read through the thread before responding; but it appears my analysis is not unique. >>>