You have to teach him to shoot lefthanded. I have the same problem as he. I have been shooting for 10 years. If my right eye gets anyworse, I will have to switch. this will put me back quite a bit in my skill level. I am able to shoot handguns righthanded by shifting the pistol over to my strong eye. But Not rifles.
Your boy is new at shooting. The wrongness he may feel at holding a gun left-handed should disappear quickly with practice. The eye is the first and most important element of shooting.
You will have less luck finding deals at guns shows in lefty guns. But most manufacturers make lefty models.