Having been teaching shooting for 20 years one thing is certain no two peoples eye sight is the same. Eye dominance can be very positive one way or the other or they can be balance with neither fully in control. This later is probably the most difficult situation as to get consistent results there has to be a consistent picture. With central vision i.e. the eyes in balance, any small influence can switch eye dominance from one to the other and consequently the gun is rarely pointing in the same place from shot to shot.
If he shot pistol successfully using one eye then try the same with the shot gun at least he will have a consistent picture. There is a small advantage in having both eyes open on some targets, but only if the eye next to the gun is in control.
Make sure that the gun fits him as a low comb which will obscure the eye next to the gun - can to a layman look like cross eye dominance.
Once you have established that it is a cross eye dominance problem and that he can shoot effectively with the eye furthest away from the gun obscured, you need to get an experienced coach to advise you on the options available to you and the best way to progress. Without this help at this critical stage you could be handicapping his shooting for the foreseeable future.
John
