People are fairly adaptable, but that doesn't mean it's the 'best' way to do something.
My son is left eye dominant and left handed; he's of course shot left handed from day one. Though he shoots very well left handed w/ rt hand bolt guns, that's currently all he has to choose from [I'm waiting until he can handle a little bigger caliber before buying him a left bolt gun].
As noted above, a lefty shooting a right bolt is no big deal for the 1st shot. While we all strive for everything to fall over after shot # 1, sooner or later it's not going to happen and a rapid follow up or two or three will be required. A lefty shooting a rt bolt gun cannot possibly make as quick and accurate a follow up shot [or shots] as he could shooting a left bolt gun - particularly if he's shooting free hand.
You've done exactly the right thing teaching your son to shoot left handed. Find another gun dealer - I suspect all this one had in stock were rt handed guns anyway?