Grandparents can have visitation rights under a possession order but that does not sound like your issue. It doesn't matter that the mother will allow you to see the children but not the father. His rights relate specifically to the possession order.
I agree with Mr. Kielich. If there is a order in place and the mother is violating it, then your son needs to seek an enforcement of his visitation. If there is not an order in place, he needs to file suit so he can exercise his visitation.
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