This has really saddened me.
Similar thing with myself, I had a relationship with a man 11 years older than me at the age of 17. Isolated, controlled me agressively, I left school to start earning for him but got in thousands & thousands of debt covering our rent most of the time cause he was self employed & was incapable of holding a PAYE job down. Believe me I could go on with the series of events but what I do know is I hid it from my family, not extremely well but they didn't know the half of it. It's good you're aware of what's going on but it took me 7 years to get out of my relationship because of his constant threats that he'd kill himself if I left him. I nearly took my own life instead a couple of years ago.
Problem is, whatever you say to him, you will be the bad guys as she'll be in his ear. Just like my mind was warped by my ex. He's at an age that I was, where you don't really know yourself, so easily influenced, the relationship you get involved in can be so crucial to the development of who you really are at that point.
The best thing you can do & like my family did in the end, was give me space & let me come to my own terms. Subtle advice may help a long the way if he chooses to talk to you but he has to decide & realise this himself. It will pain you to watch, he may push you away, especially if it goes on for years but he will get there if he has a good head on his shoulders. I nearly married this guy at 19 yrs old, my dad was petrified that I'd said yes to the proposal but when it came to it I kept postponing, knowing the sick feeling in my gut was telling me something.
Your son will learn from perhaps talking to new people he may meet wherever he works, that helped me a lot. Started working in the city, making friends at work, gaining an understanding of others people situations.
He knows, believe me, he can lie to himself for only too long until he'll break & realise.
Patience & space is my advice & don't give him & definitely her reason to be able to say a bad word about you.
X