why give a damn about some nerds who may or may not rage at you for losing a normal game
Because they're people. And through incompetence, ignorance, etc., you may hurt or disappoint them. It doesn't matter whether they rage or not, or even if they talk at all. I know that I'm the one who failed to secure baron, or who smited red buff at the start rather than the end and got counterjungled because of it. I know I misread the enemy laner's actions and that he clearly had a ward where I was waiting. I'm the one who chose to focus on getting our midlaner fed when he ended up just being good at laning and terrible at team fighting. I know I failed these people. Sure, they make mistakes too, but that's beside the point. I messed up. I let my team down. (And it sucks that the only way to really get better is to keep playing even knowing you're going to make mistakes.)
Why is it important your son play jungle? Is it that he says he wants to jungle, but has trouble going through with it? If it's just that you (or his friends or whoever) want a jungler to queue with, it seems like simply letting him play his comfortable lane would be easiest. (It's easier to drive around a mountain than over it.)
LostFrost's point about doing custom games for jungle clears helps me a lot. (For me it's more PvP anxiety than ranked/role, but still fairly applicable.) Knowing that I'm able to do a full clear on my selected champion, by a certain time, with a certain amount of health left, and checking on my lanes (moving camera, not just minimap) every 5-10 seconds helps me.