Roommate problems

So, me and my SO have had his best friend living with us for about nine months now. My bf got a job in another state and I'll soon be joining him up there. Our roommate and him have been friends their whole lives and they're like brothers but the roommate has been through losing both parents a few years ago and isn't the most stable person mentally. How am i supposed to tell him we're moving and he's out of luck??