Social Services are overwhelmed & at this stage this case is unlikely to cross the threshold into case management.
Start with WA & take their advice at this stage.
All of this must be distressing for you, a shock to find out that your adult is behaving so badly & hurting the mother of his child. The seeds of this behaviour lies who knows where, but it is not acceptable or excusable.
I asked in pp if your son is aware that his partner & baby are with you. I am concerned that once he finds out how much you know about his ‘secret’ behaviours that this may precipitate some form of crisis or rash behaviours.
The NDV helpline - see image above for number - can give you sensible advice about protecting you & your home, when to call the police if you become concerned about your son & his behaviour.
From what you say, your son already has some MH challenges e.g. anti-depressants, which does not excuse his abusive behaviours but he may already be receiving MH treatments that could help him if his abuse comes to light.
Other posters may know more about what I suggest next: it is not unreasonable for you, as his mother, to contact his GP to express your concern about his behaviour. No more than that. The GP may take note of your comments.
Again, it is Women’s Aid who have the breadth & depth of experience in these matters.
I wish you all the best.