Thankyou so much to everyone for your advice and support. I can't tell you how grateful I am.
Young lady and GC now safely with her family. I have spoken to her and also to her own mother who is very thankful and has been very sensible.
We made a phone call to local police who basically said my son didn't have a leg to stand on as they would arrest him for domestic abuse if he approached them. We also saw the GP and had the situation recorded in GC's notes before they left.
I have spoken to Son. I have lied about my involvement with the flit which I feel terrible about, but it seems the only way.
Partner contacted Son earlier, explained in a message her reasons for leaving, and said she was willing to arrange regular video calls and visits with GC notwithstanding her wish that the relationship ends now.
My son is in shock but tells me he knows he behaved terribly. It seems he "snapped" and he is full of remorse and accepts the relationship has ended as a result. They have both told me separately they only stayed together because she became pregnant. They are wildly unsuited, both volatile and extremely impatient and I have witnessed so much shouting and name calling on both sides.
This is not to excuse anything, just explaining what it's been like.
I think we're all still in shock. I am of the view now that I may have prevented the situation escalating to the point where my son may have done something we could never forgive. So I am at peace with my role, although she sad it's ended like this . Horrible saying goodbye to little GC but I know his mother's family are good, loving people so that's a bonus in this sadness.
Thankyou ALL again