Perhaps the best thing to do is to just be up front to him. Give him a call on the phone, and say, "Hello Mr. ____, do you have a moment to chat?" And then after exchanging pleasantries, just say, "It was so nice to have your wife come and visit the other day. There was something I wanted to bring up, though. My son is not comfortable with exchanging hugs with people he doesn't know. I don't mean to hurt your feelings, or anything. It's just the way he is." Or something to that effect.
It's so sad that everyone has to walk on eggshells when sometimes, just coming out and saying it is so easy, after you look back on it, you wonder what you were even worried about in the first place.
Just my two cents anyway.
Take with a grain of salt.