Being right handed and left eye dominant I sorta know where he is coming from.My nephews have the same problem and we just taught them to shoot left handed from the start.
He might try useing a pistol.Its the easiest I have found to use my left eye and still shoot right handed?(Might be a problem useing a pistol in Cananda though?)
Another option is to trim the stock down,in essents take the cheek weld area down till he can rest his head down far enough to use his left eye.It might not work with a gun with a LOT of recoil but I know it works with the little critter-getters.Here is a pic of kinda what I mean.
Sorry I dont know of any scope mounts but am sure it could be done.