Shoulder/chest rigs....
The Bianchi UM series(M12/UM92I/UM92II) can be quickly converted from a belt rig to a shoulder to a tanker(chest) with add-ons. The holster includes a barrel rod.
If the military holster isn't ideal, see the holsters from Simply Rugged or maybe Lawman Leathergoods.
I'd suggest you or your son contact the holster firm first to insure they can make/provide the style you want & when it can be sent. Many custom or special designs take a few months to deliver due to demand or limited resources.
Clyde