I realize that I am also chiming in a tad late here. I would agree that as young as your Grandson is, he should have no trouble continuing to learn to shoot lefthanded, using his dominant eye. I'd not sweat the idea of how hard it might be to find a lefty bolt rifle, as there are other excellent choices that will work perfectly for him. One of the best that comes to mind is the fine Browning BLR, which comes in a lot of great chamberings. The lever is amidextrous and so is the hammer/safety.
A Marlin Lever gun would also work, but it does have a second safety to contend with, although with proper teaching he could just as easily hunt safely (as I do) without using the HBS (hammer block safety found on the Marlins made since 1983). The safety can be used while loading and unloading, but he could simply rely on the original half-cock safety while he hunted.
Nothing wrong with a nice lefty bolt gun, but there are also some very fine lever rifles and single shots, like the Ruger #1, that can be used just as easily by a lefty as a righty.
Best of luck!!