Sorry I don't know the answer to your problem, but I would think he could find some special condoms if he really needs them. I think width would be more improtant than length in keeping a condom on.
Ill let a guy help you with the answer to this one but I do know that if they are too big or too small they can come off. If he seems to just be buying large for the namesake of it, you might want to consider bringing your own in his size