I think Julie F. made some excellent points.
Things like his body shape/type and his coordination are observable. But how is it they know that he wears a pull-up at night? If your son said something to them or to the boy, that's one thing. But if you are divulging too much info, you might want to re-think that since these people are not going to be a source of support or friendship to you.
I think you can repeat (ad nauseum) that he's been checked by the pediatrician and his body will catch up on its own without intervention. Then change the subject. If your answer gets "old" and repetitive to them, maybe they'll stop. If they don't, you can say, "Perhaps you didn't hear me. I don't find this criticism helpful at all." Then leave.
If they are saying anything directly to your son, I think you absolutely must step in and put a stop to it. "Matilda, let me be very clear. I think it's nice when neighborhood children can play together rather than only be engaged in organized activities, but if this criticism and constant correction of my Billy continues, I'm going to stop the play dates because they are becoming detrimental to his well-being. He does not need to be bullied in the neighborhood and I will not tolerate it."
I'd let the mentoring thing go - if the other boy is a natural leader (or if he needs a boost at becoming one), that's not a terrible thing. But if he's constantly correcting your son and your son is no longer enjoying it, then you teach your son to speak up. If they are at your house, you can comment along the lines of "Gee Tommy, it's nice that you want to teach Billy how to do things your way, and I think it will be nice when you let him return the favor by showing you the many things he is good at, such as X and Y." If he keeps pushing, you can comment that sometimes pushing so hard just pushes friends away, and he might want to think about that. But this kid is the victim of his mother and grandmother, so don't think you can change him in 10 minutes in your home or yard. Just pity him. He's going to have a rude awakening when he's not as good at stuff as his mother and grandmother have led him to believe. And be thrilled that you are not this child's teacher - these are the kinds of adults who drive good people out of teaching!
If that doesn't work and if your son is not happy (that's the key), then enroll him in something else OR just set up play dates with other kids in his class. If the neighbors comment, you can say, "Yes, Matilda, my Billy he has a play date with a boy across town. They really get along great because their talents are in the same area. His friend Johnny is just so easy going and kind." Repeat ad nauseum.
Try to separate out what comes across as criticism of your parenting - annoying, but not your kid's fight - and what is just their annoying personality quirks which do not require your intervention. Sometimes you just have to smile, shake your head, and walk away. The good parenting plays out over time, and you'll see your child blossom in the future because of your willingness to accept him for who he is.