Hi whogoesthere. I'm so sorry that you haven't seen the GS you were looking after every week for 3 months because your son's marriage has broken down.
It's going to be difficult for your son to gain access to his son if his estranged wife refuses to speak to him and as others have said, it is through your that your relationship with your GS will continue.
If he hasn't done so already, your son needs to get legal advice and employ the services of a solicitor to contact his estranged wife so that his own relationship with his son is disrupted as little as possible.
Whatever has happened in their relationship, it's unfair on you and your GS for the relationship you have to be denied. The break down of a marriage is heartbreaking and while I can understand your estranged d.i.l. perhaps feeling unable to engage with you, I don't see why arrangements couldn't be made with his other GP's, especially as they are now living with them.
I do wish that adults would behave like adults and put what's right for their child(ren) above their own feelings, especially when it comes to ensuring that a close relationship that has developed between children and their GP's is maintained.
I hope you get to see him soon.