Understand it from grandma and grandpa's viewpoint...
For two years, you and mom and junior lived with the grandparents (?rent free?) and then mom continued to do this for another 10 months...and continues to use her parents as free babysitters and a free taxi service for your son.
Is it unusual that they might assume they have a role in determining some of the choices they have made for him? No.
Should you two be grateful for what they have done for you?
Yep.
Is there a gracious way to make them understand they are overstepping their role?
Perhaps not since you accepted (and continue to accept) their services and implied that they were part of providing support to your son.
Gail