Personally, I'd be leaning toward a hot loaded .45 Colt over the .357mag. In the leverguns, you can push a great deal beyond what a revolver can handle, so you'll see that factory loads are left wanting.

The .357mag will do the job, and will have less recoil, call it a 75yrd-ish rifle if he can shoot it well enough. The .45colt will anchor them better, about the same range, 100-125yrds if you find some hotter loads for it, and it still won't have any real appreciable recoil.

You'll probably want to cut the buttstock to shorten the pull down for him, and cut it short enough to account for a buttpad to take the hard edge off of the kick. Neither will kick hard, but a pad won't hurt.