He needs to learn to use his dominant eye, and shoot right handed.
I am a typically right handed person that shoots left handed because I am left eye dominant. I shoot very small groups.
Suggest that he practice shooting a rimfire right handed. If he is right eye dominant, he will probably pick it up pretty quickly. Trigger control requires more focus than physical dexterity.
He needs to develop good trigger control with his right hand and use his right eye. I don't know what he is shooting but if your going to learn to use the other hand and eye, a rimfire is much better for developing good habits.