I would def get legal advice. I think it will depend on custody arrangements in place ie if she has full guardianship and custody then the decision can be made by her.
Also it may be worth you visiting ur DSS to work out how he is going and where he is at. It could be very traumatic for him to have been moved and again if he goes to you. It is likely that he has viewed being left there by his mum as a rejection. It may be worth consulting a child psychologist or child specialist agency just so whatever action you take is done with his mental health at the forefront. Do you know if he has been getting help somewhere for his difficulties?