Suitable long gun for someone who has weak hands.

[JuglansRegia]

M1 carbine, if you can find a USGI version in decent shape for a decent price.

 

[Member 53328]

If your son is like mine was, also on the spectrum and perseverates on subjects, handing him a gun he uses in games and teaching him to shoot it will likely start a process in his head.
If he isn't strong enough to handle it, you can point out his characters are strong and teach him exercises to get there.
My son looks like a blade of grass, like he will blow away in the wind. But he's a surprisingly agile and strong one. Part of it was teaching him to shoot my 338 Winmag, his favorite rifle.
I would encourage you to get him off gaming - if he's playing FPS games, it induces a great deal of cortisol in the player, which wreaks havoc on muscle development and fat deposition. This is critical during the physical maturation years of 12-21.

Great post.