I think the only thing you can do is support your son right now. He needs to sort out the situation, because he will be required to support the child, unless of course he can prove that it is not his, but presumably you know it is his child.
We might all be better able to help if you told us a little more of the background.
You say the child's mother won't talk to any of you, and you do not know why she will have nothing to do with you. Ask your son, if he has even the faintest inkling of the reason. I'm afraid someone on your side of the family must have done or said something that the child's mother took offence at - she may have totally misunderstood something that was said or done.
I can't say I think it will help, but you could write a letter, apologising for anything you could possibly have done to hurt her. Make it clear that you are not aware of what it could be. Appeal to her love for the child - children should know both their father and their father's family.
If you send the letter by registered post, you will at least know that she received it.