I am sorry, but this is going to sound really rude (I guess it is). But are you a really quite dense?
Do you not realize that your setup is highly unusual? Do you not realize that even the fact that your son lives at home rather than in student halls or bedsits, or student flat shares is unusual?
It is NOT unusual to have a part time job, to have student loans. It is however unusual to live with your mum.
YOU are failing your son.
He should be doing what most other students do: Move out, get on with his studies, find part time work, apply for student loans and LIVE HIS LIFE INDEPENDENTLY OF MUM.
I am a landlord. I have 4 students renters. All 4 work. All 4 have student loans. All 4 are graduating this summer. The have been living together for three years, managing life, work, education. Just like I did when I was their age.
This is what you should encourage your son to do.
I just dont understand how you can be such a silly doormat. What are you getting out of it? Mileage for thread after thread of self induced drama?
Stop being daft, stop wailing "but where are they going to go".
Find a solution! Or let him find a solution!