I'm a lefty and I've taught myself. When I have a bad shot I try to think what I might have done different. I'm learning to figure these mistakes out and my shooting is much improved.
I don't know what to suggest to help except perhaps watching him shoot looking thru a mirror. Perhaps you can get a better perspective that way...
You didn't give the boys age - it will be much harder to teach a young boy compared to a teen. It's hard for the younger ones to concentration on form I imagine.