Two in-law grandpas (one is the father of the mom, other father of the dad) were in the office and they were just saying how amazing it was that their grandkids were sooooo different with them than with the mom and dad. They're pretty much angels with the grandparents, and when mom and/or dad come back the kids just go nuts and the behaviors went to bad quite often. And these are easy child kids. Kids just seem to act better for grandparents than their parents, I know mine both do. Yes, he probably relaxes and laughs more with grandma. You're the mom - who has to make sure he learns responsibilities, goes to school, does his homework, cleans his room, doesn't do a whole lot of things that society frowns on from scratching where he shouldn't to not stealing etc. And it's up to you to be the bad guy if he's not doing what he should. Grandparents get to be the good guys mostly, take them fun places and buy them things and when they aren't behaving right take them home to mom because "the little darling is having a rough day".
I agree with wyntersgrace - I would be very concerned with a pediatrician who would discuss me that way with my mother. I would be looking for a new pediatrician that would support me, and if he did think that way discuss it with me and not someone else.
