Hey go easy on us leftys. I am a lefty and I teach industrial arts to predominately right handed middle school students, try that. Any ways, use his opposite shooting style to your advantage. Stand beside him and mirror what he is doing. This may help you see where he is going wrong. Also if he needs help have him mirror you. If you stand behind him it is going to look totally strange that is why I would stand to the side.
Secondly, I would start at the root. Look at his bow and make sure it is tuned. If it is, check his form. (Same Knock point, good follow through, ect.) If all else fails try a new sight. Find a good one you can afford and sorta talk it up building his confidence in it.
Good Luck!