• Myopia doesn't affect 3D at all. it's just a focus problem, far objects appears blurry. But 3DS should be place near the player. And usually, Myopia people wear glasses.
It doesn't affect seeing "far object in 3D on the 3DS", it's the 3DS position itself which define the light distance between the object and the user.
• Astigmatism should depend on the strength he has. I think it usually affect little details, like letters. if he has the same strength to both eyes, then I guess each projected image should be interpreted the same way, so there's a chance he can see 3D, but maybe only on big objects.
•Strabismus is the one I fear the most. 3DS send each image straight to the player. If he has 1 or both eyes not seeing the image as they should (supperposing each other) he will see 3D the same way you do looking at a 2D video of the 3DS : you see the 2D image with ghosts effect.
Again, it will mainly depend on the strabismus strength. some people have both eyes really far apart from each other, while other only have 1 eye slightly away from the center.
The best way to know will be to let him test on the real hardware.
I guess that if he already has troubles seeing 3D on movies, he will have with 3DS too. I guess his strabismus is too much pronounced.
PS : don't take all that for granted, I only commented on my own knowledge of each pathology. I'm not an optician.
As I said, the best way is to try.