Hello, I am a solicitor with 20 years experience. I will try to answer this for you.
I assume he doesn't pay rent and has no interest in the ownership of the property. In which case he is simply someone you allow to stay on your house as a guest and has no legal status. You can ask him to leave (perhaps give him a little bit of notice although he is not legally entitled to it).
If he refuses to go you can simply change the locks when he is out.
My name is ***** ***** I have been a solicitor for more than 30 years.
To avoid your son using family law to cause problems you need to give him at least 28 days written notice to leave before you change the locks - otherwise he can obtain an Order allowing him back in temporarily and that is not something you wish to risk
Please ask if you need further details