@Tigerzmum I’m really sorry you’re dealing with this. You’ve done far more than most parents would, and your son is now a 24‑year‑old adult who is choosing not to take responsibility for his own life. Its not coercive control but it is financially controlling and you can do something about it, but I get its daunting.
For clarity, and assuming this is England: He is NOT a tenant. He is an excluded occupier, which means he has very limited rights in your home. You are fully within your rights to withdraw permission for him to live there. You’ve already given him verbal notice a year ago and written notice two weeks ago unless I have misunderstood and that is more than reasonable.
If he refuses to leave, you can change the locks while he is out. This is legal for an excluded occupier.
The police cannot force you to house him. They may attend to keep the peace, but they cannot make you let him live in your home. If they turn up again, calmly state: “I have withdrawn his permission to live here. He is an excluded occupier. I do not consent to him entering my home.”
You must not physically remove him, but you can refuse entry and arrange a time for him to collect his belongings.
On the benefits side: He may not qualify for JSA due to lack of NI conts paid, this is true, but even if living with you he can claim Universal Credit as long as he is no longer enrolled as a student. The student loan only affects UC while he is officially studying. If he has dropped out, he can apply immediately. He is choosing not to, that is not your burden to carry or at best, has misunderstood. What I want to know is where does he get his money from for drinking - is there any chance he is lying to you about the benefit?
You are not being unreasonable. You are not responsible for funding his adult life, cleaning up after him, or sacrificing your retirement because he refuses to grow up. Wanting peace in your own home at 66 is not selfish, it’s basic dignity.
You’ve given him every chance. It’s time for him to stand on his own two feet and for him to understand you have options too. He needs to start considering his own well-being and that may include his mental health. At 24 you cannot do it for him, you can support him if he chooses to go down that route, but you don't have to live with the consequences if he does not.
Having said all of the above, I get that your son could end up on the street if no-one takes him in and then the flying monkeys will descend. So it it was me, and this is entirely your decision. I would sit him down, tell him that he needs to start taking responsibility as an adult because you cannot afford financially, emotionally or physically to support him in the way he has been living since University.
Tell him he is entitled to UC, and needs to apply for it NOW (and you need to see proof) and start paying rent (if you would allow him to stay on that basis) however, I would be inclined to tell him about UC, tell him to claim now and then
Step 1: give him a formal quit letter including a sentence along the following.
“You are an excluded occupier. Your right to live in my home is withdrawn. You must leave by [date]. After that date, you will not be permitted entry.”
Step 2: Give a reasonable deadline
7–14 days is typical, but you've already given 12 months.
Step 3 Change the locks
Do it while he is out. This is legal for an excluded occupier.
Step 4 : Arrange belongings collection
Give him a time to collect his things from the doorstep or garage
At a time you feel its right, you can consider telling him that once he has started taking responsibility and addresses his own Mental Health, financial and emotional needs and has shown some accountability, you may reconsider in the distant future allowing him in the house as a visitor.
Good luck OP, this is a very daunting situation to be in.