Left Eye / Right Hand dominant here. I am equally proficient with my left hand using long guns, but not pistols.
Originally I shot long guns LE/LH, and pistols LE/RH, but that leads to a bunch of problems. Shooting pistol that way weakens your stance because your wrist isn't straight, and that is reflected in your groups, and unfortunately shooting pistol lefty doesn't work for me. Also should your son ever compete in shooting games like 3-gun, consider how the gear is laid out if he is lile me and shoots long guns LE/LH and pistol RH. I tried and ended up with a holster and all my mags on one side of my body. It felt ridiculous and looked even crazier. That following winter I dry fire practiced RE/RH religiously and I am now RE/RH shooter, despite being LE dominant due to a RE stigmatism. Saves on hassle, gear, and lefty specific rigles with poor re-sale value.