Oh I know, and I do try to be patient, honestly. It's his habit of yelling at me (or at least in my general direction) when he doesn't understand something, and the REFUSAL to admit that he doesn't understand something, that gets to me.

I once gave him a computer lesson with the proviso that he was not allowed to interrupt me and start yelling if there was something that didn't make sense to him, and if he did I was going to walk away. That actually worked quite well. I find it all very stressful though.