He's hurting, give him space.
This is likely to feel similar to him, as a miscarriage does to a woman.
He had absolutely no say at all in the decision of if his child is aborted, or brought into the world. He wasn't even asked his feelings on the subject, he has just simply been told after the fact "Oh, you created a life, and i decided to end it without even letting you know so you could say goodbye to your own child".
To add to that pain, he then finds out his own mother knew about the baby, and the plan to abort it, and didn't have the decency to tell him!
You have knowingly denied your child the chance to know about their own child while they still existed, and process that and come to terms with the idea of termination, and helped deny him the chance to "talk" to the baby and say goodbye and make peace with the decision.
You were absolutely in the wrong to not tell your son, your loyalty should have been to him and his feelings as his mother. How would you have felt honestly in his shoes? If someone had made the decision to abort YOUR child without you even being allowed to share your wants and feelings? It was clearlyfelt he would possibly want this child given he reasoning of being affraid he would try to change her mind.
His gf is quite frankly an awful person. Shes happy enough to commit to a relationship with him, happy to consent to have sex with him, knowing theres no 100% reliable contraception, willingly accepting the possibility of creating a child with him, and then doesn't even have the respect for him to let him know the baby exists or ask his thoughts and feelings and decides to abort (yes its HER decision, rightly, but he had a right to know about the baby BEFORE the abortion) then doesn't even have the decency to keep it a secret, and relieves herself of her own guilt by unburdening herself of the secret and turning his life upside down devastating him? Thats quite evil actually.