Contacting the mother or the other grandparents would only cause them upset and serve to destroy the relationship with the son
If there are welfare issues with the children you believe the school or whoever referred the child for help does not know, you should contact social services or child protective services.
The fact that the child is being given help would suggest that Dad is trying to do his best for them so what has Dad said about the situation with the mother?
The things he has said about the mother has he said in front of the children?
Another point, just because you believe the children are being kept from you wrongky doesn't mean Dad has done that to the mother and other children. Dad has custody after all.
Also, for whatever reason, son and dad have not been close and not had a proper relationship, Dad may be trying to protect his children who are already struggling, from a relationship he doesn't feel secure in yet.
As previously said, the best way to have a good relationship with the grandchildren is to have a good relationship with their father.