I guess I'll be the one to go against the grain and say, make him
change his eye dominance.
It really isn't that hard, didn't you see Fire Birds with Nic Cage?
But seriously, when I was a kid in scouts and first shooting a BB gun, I got SO frustrated because I was left-eye dominant and right-handed. When they figured out what I was trying to do, they said, just close your left eye. I did that for awhile, and eventually my brain just decided, "okay, you are right-eye-dominant now." I now shoot handguns and long guns with both eyes open, right-handed, right-eyed, no problem. I can now do the Miles test or Porta test or what-have-you and am very firmly in right-eye-dominant territory. When I get behind an AR, my right eye instantly finds the front sight through the aperture sight, without even thinking.
He doesn't have to even close his eye all the way, just lowering his left eyebrow and keeping his right eyebrow high helped me. Or scotch tape over the left lens of his safety glasses.
I think a 13-yo would be better served over his lifetime to learn to shoot RH/RE. It will save him a lot of headaches down the road I think. I really don't believe eye dominance is as ingrained as handedness. And I even think handedness is not as set-in-stone as people think. If you are forced to use your non-dominant hand such as people who
have lost limbs, you find out that after a period of intense frustration...eventually you can get the coordination and dexterity in your non-dominant hand pretty close to what your dominant hand used to be. The same goes for eye dominance, and it is wayyy easier to switch eye dominance at 13 than handedness at 31.
IMHO