i am going to assume for the purposes of this post that you have done nothing to cause this. So that being the case, in your boots i would write a note to him for every Christmas and birthday and any other significant events and keep them in a safe place, plus some photos and whatever else.
I would open a bank account in trust for him and place a sum of money for each birthday and christmas.
I would make the letters part of this trust. I would also set aside an amount for a tracing service
I would then have a will written up and make him a beneficiary of a significant part of my estate, maybe your sons share, and, ensure that there is a provision in the will to pass to him, a letter that will be more significant as time passes.
I would also include in the will what i felt about losing him but not be mean about mum.
Once i had done all of this i would politely write to his mum and express my sadness but accept her decision. I would tell her that my door would always be open should she change her mind.(I would keep a copy)
I would let her know that significant provisions will be made for him in my will, and that upon his majority the trust fund will be available to him and your letters for what would have been sent to him, had this not been her decision.
I would also let her know i will make no criticism of her anywhere.
That might bring her to her senses if you are innocent of wrong doing. And if not he will know you loved him when he is grown.
I have given this advice to my embittered mother who cannot see the harm my brother has caused, and has made things so much worse for herself