It seems I am always having someone moving in or wanting to move in with me. All my generosity that I've provided over the years have now put me in a lot of debt and guess what.....I don't think it helped them at all. I recommend setting a date of move out or rent payment and STICK TO IT!
I know it is hard because he has a 15 year old, but you can always state that the 15 year old is welcome to stay as long as he follows rules of attending/finishing school, attending church, etc. and upon graduation moves out. You have to be stern, but in the long run, everyone benefits. Your step-son may be having depression and staying at home with no goals, no responsibilities, no interaction with peers will only make it harder for him to get back out in the real world. Good luck and God bless you!
B.