I agree with previous poster OP, after all this time it might be good if your DS actually discovered you in tears, he's bound to ask what's wrong, and then you could tell him that you really are worried about the fact that he's not doing things that young men of his age would normally do. That while being in a relationship with an older woman might make him feel grown up, but in actual fact, it appears to be stopping him trying new things, because he's spending all his time with his GF, and that's not normal in any relationship. If you and his Dad spent time apart doing things that your son was aware of, you could perhaps point out, that you didn't spend all your time in each others pockets. Maybe you could point out that a woman of her age, has had 11 years to go out with friends, travel the world, have lots of different boyfriends, try different jobs, etc. which is fine and the way it should be, but you are worried that she seems to want to stop him even seeing his Mum, let alone spending time with his mates, or playing football. These are all things that have just sprung to my mind OP, but knowing him as a Mum does, I'm sure you know the sort of things that might get him enthusiastic, other than this horrible, manipulating bitch. Another example, has he ever shown any interest in playing golf or cycling - sports that might appeal to him, does he know what he wants to do when he leaves college, if he's mentioned a type of work that he'd like to do, you could suggest something he could be doing to prepare for that. I'm sure if you put your mind to it, you can come up with some more ideas.
As for the fact that your DS didn't have his phone at college with him, and is now having lunch with her every day, I would say that she's probably pointed out that he'll be seeing her at lunch time, and she'll pick him up after college, so he doesn't really need it, and of course terrible interfering Mother might be interfering with his classes by her frequent texts, blah, blah. Another way of controlling him, making sure he doesn't speak or text anyone that she doesn't know about. Just so you're aware, I've experienced this sort of controlling behaviour, and how these people manipulate others into thinking that the only thing they need in life is them, which of course any 'normal' person knows is totally wrong, but love bombing them, makes them feel that they can't possibly complain, because who can complain about being made to feel loved?
Sorry this is so long, I guess having read all your posts, I felt I might finally have something useful to contribute. I feel confident that he will EVENTUALLY see the light OP, it may take months, or even years, but it will happen, so in the meantime, you need to look after yourself, maybe find some new hobbies or friends, to help take your mind off things in the meantime, but just make sure that he ALWAYS knows you're there for him. Sending a hug your way!