I'm late to the party but have read all three threads. It's heartbreaking and I feel for you. There's much criticism on these threads but what seems to be forgotten is that there are adults in this situation and they react as your DS and these adults have far more life experience than a 16 year old boy.
I'd be happy and loving towards him but I'd get on with family life. If I was seeing him at a game, I'd happily chat about all the great things you'd all done without him, eg cinema and meals out etc. Tell him that you missed him there but he's living with his gf and you don't want her to take it out on him. Yup, casually turn it back on her - in the nicest way.
I'd have to stop with the driving lessons and everything other than his phone.
Remember, when talking about what's happening, make it about the actions, not about her. You're also entitled to tell him how you feel about reading things about yourself on FB, eg hurt, upset.
Good luck OP, stay strong.