*Life is tough for young people these days. I am sure finding somewhere to stay or flat share with someone you like is difficult.
Let him stay and charge a reasonable rent. He won't be there for ever.*
This was kind of what I thought, maybe combined with an earlier suggestion that when you're using it, he has to sleep elsewhere, be that a B&B, a friend's sofa or a travelodge. That way the flat is being lived in, he's able to save a bit but is getting used to paying rent and bills, and you still have the convenience of the flat - and him getting groceries, bread and milk in for you when you do go up. As long as he looks after the place, and keeps it clean, I'd be happy to help him out. My daughter, a student in London, has to pay £260 per Week for her rooms, it's absolutely crippling for her. If I had somewhere to live there which I wasn't using all of the time, I'd definitely let her stay.