My son is still studying (aalim course) Inshallah he's starting his last year in January.

Question

My son is still studying (aalim course) Inshallah he's starting his last year in January. I've been paying maintenance consistently for my grandson as my son is still studying. The mother has refused that any of us should see my grandson, as she is recently remarried and her husband says we can't see him until he is of an age where he understands why the parents got divorced (her reason given). We were seeing him before this, for an hour, once every few months. She has returned all my grandsons maintenance money that I've paid, since he was born. He is now 2 years and one month old. I will continue to pay the maintenance as it is his haq. She can continue to return it, if that's what she feels she wants to do. We do not communicate. My questions are: 1. Do I keep this money until he is baaligh and then give it to him? It was not her money to return. He is rightfully the owner. 2. Does his father take ownership of the money? I'm really at a loss as to what to do because it is already a large amount of money and it will become larger as the years go on.