Marle,
I had to do that last year. This was my older, sober son, but he was bumping up against boundaries and I felt as though he was taking advantage. It was just TIME for him to get his own place, but he was very reluctant. It was difficult for both of us, because he didn't think he had "done anything wrong."
In my experience, getting him to take the first step was the most difficult. We finally sat down one night and talked about money and budgets and what he thought he'd like to have at his own place : cable, internet, big screen TV ... you know, all the important stuff LOL and it got better from there. He saw his own income and his own stuff coming together on paper.
Good luck, big hugs from mom to mom. As I've said before, this mom stuff is NOT for weenies