Although it's hard to judge from your post it does suggest that DDs partner is a domestic abuser- controlling behaviour, isolating victim from support networks etc all in the name of "love". Whilst I agree with other suggestions of looking after yourself in this new future, at the heart of this is the welfare of your DGS. If his dad is and continues to have independent access to his son then at least he can monitor him and inadvertently your DD. However if this boyfriend is a domestic abuser then he will force this to stop. So either DGS's dad, yourselves or both need to go to court ASAP to ask for direct contact with DGS. It's not too difficult or expensive as you might expect. Also is DGS to remain in his current school? If so that's another protective factor. You can't do a lot about your DD at the moment but you need to make sure DGS is safe, loved and cared for appropriately.
When will this wind ever stop?!
Recommendations please, for a stopover on the way to Loch Tay
To think that London, or anywhere else for that matter, does not belong to any one demographic