Thanks for all the advice and tips. You're all lovely 
I've looked up some grief counsellors and there's a support group for kids quite near. Initially LB had a couple of sessions with a counsellor back home but he didn't like it, and probably wasn't really ready to deal with things. I will chat to him about the options when he gets here though.
He's not really being asked to leave my BB's house, we gave him as many options as we could, if he'd chosen to stay i would have stayed in my rental and used my savings to help BB buy or rent something bigger. I'm buying my first house ATM so I'm hoping it'll be good that LB is moving at this stage because it'll be us moving into our house rather than him moving into my house. I'm hoping that will make it seem like he has a place of his own.
My work is trying to be flexible but they are used to, and expect, long days from me, usually I start around 7:30 and finish at 8:30/9, but that won't really be an option, so we've sort of agreed to me having a home office set up so I can work mornings and drop him to school before work and then evenings once LB is in bed, and someone is going to cover my on call police station weeks from Jan - March so I won't have antisocial calls until we figure out who can watch him if I have to dash off.
I think the main advice I'm getting is just be prepared for him to challenge the boundaries a little and try and keep talking.
As bad as the circumstances are, I'm looking forward to having him around, and BB and I are saving extra in the month so LB can travel to see BB once a month if possible. They're both coming up tomorrow to spend the weekend, see the new house and all try and talk through things. I think both my BB and I struggle a little to know what we should say to him and what not to say, he's still so little to us, but has definitely lost his little kid-ness this year. He's been really brave but our instincts are protective. I guess he will let us know if we are treating him with kid gloves against his wishes.
Sorry to ramble again. Thanks so much to everyone who has taken the time to reply xx