So, she is in debt then?
You pay half of everything materialistic (clothes, school trips etc) but what about maintenance for daily household living? I don't mean to sound mean, but I am wondering if you pay anything towards daily living costs? Maybe that is why she has had to take in a student/lodger? If perhaps, you increased any weekly maintenance, he can have his room back. I know it's probably not just as easy as that, but it may help
What does your ex say about this? I can see that having 3 other children living with you may mean it would be a tight squeeze with your son there also.
I am actually quite shocked his mum can do this TBH - but as previously written there is no law to say he cannot sleep on a sofa, sorry