Bf wants me to move in but he has a roommate

So my bf has been talking about me and my 2 daughters moving in with him. It's a huge step but right now would be perfect since I'm not currently working (I'm a preschool teacher) and school is basically on hold (to be a nurse). This gives me time to figure out childcare for when I do start work and which school they'll be attending and kinda get a routine going. I would have to apply other places because my current job is 1.5 hours away. I would have a lot of free time to figure everything out before all the craziness begins. There's one problem though. My bf has his best friend living with him. This guy was only supposed to be there 6 months max but it's been 2 years. My bf didn't mind because he was living alone and besides, they never really see each other. But now he wants him out but he doesn't know how to tell him. He has a gf and they've been looking for a place to move in together for the last 6 months but it seems like they're not really pushing it or making it a priority. How can he ask him without him getting offended or upset?