You do not say how old your DGS is now. I deduce he is between 12 and 14, so his views on where he wants to live will be listened to, and usually, taken into account, unless it is unrealistic.
If SS get involved, encourage him to talk to them honestly about the abuse he suffered when he was with his Mother and step-father. It is unlikely they will force him back, because they cannot physically force him to stay there, nor can they stop him walking out again.
I think you could start by talking to the NSPCC, Barnados or Childline as already recommended.