You need to remember that what you have described is (also) attributed to abusive, predatory, people and the co-dependent personalities upon which they prey.
Your Son could very well be in what is refered to as an enabling relationship and (in any case) only he will be able to remove himself from it by hitting bottom and finding the courage to resist her.
I feel your pain--for I have lost my Daughter (who was my closest friend) to a man like this. He wields a power over her that makes her forget everything she was ever taught about self-respect, independence, and self-esteem.
In order to have him--she created a needless situation between the two of us. She had to remove me from her life--in order to have him and did so without even batting an eyelash. I tried to include him and she excluded me (instead).
She is behaving completely out of character (much like your Son)--but there is nothing that I can do--until she determines to reclaim control of her life and re-establish relationships with the people who truly love her and want her to think and decide for herself.
I would only be sinking to his level and playing into his hand--if I tried to influence her choices--so I have taken a large step backward and removed myself from the equation.
I have decided to have faith in my Daughter's (innate) intelligence and independence and wait--until the seeds I took so much time and care in planting begin to take root and grow. This is her first romance and the honeymoon will eventually end.
I may not agree with her current choices and I definitely have issues with her disloyalty and deception--but I must believe that one day she will come back to herself and (even if it is too late for us) regain her self-respect.
Peace and Love on the path of your choice...
Blessed be...