If you can, take him to store which has both left and right handed instruments (preferably of the same or similar type, ie 2 squire jazz basses, set up similarly). Show him how to play an e minor scale and have him try that on both and go back and forth a few times. My guess is one will feel more natural to him. In which case he should go for that one. But if there's not much difference, he should go with right handed, because it will be way easier to find instruments and also sell them later.
I'm someone who's right handed but also not quite ambidextrous. So I do tend to do a lot of things with my left hand. I've tried picking up left handed guitars and basses from time to time and they always seem like an alien instrument. I can't say if the reverse would be true, because our left and right brains also tend to control certain tasks and thought processing, so there are a lot of factors. I will say left handed drumming is kind of cool because it opens you up to new patterns, and though I've only dabbled with drums, I always liked setting up left handed.