Does my son have to leave?

My son started buying his flat with his girlfriend a year ago at the time he had a job , he lost it through forgetfulness (caused through a car accident he badly damaged his brain) he got another job 2 months later and lost that one too , he is on jobseekers allowance now which is only £59 pw , so he cannot pay towards the bills as much as he would like , his girlfriend wants him to move out back to my place , I think he should stay until the place is sold , as his name is on the mortgage and all other bills , I will let him back here , but not until the flat is sold , does anybody know what the rules are for this dilema , please only genuine answers ,as this is very serious.