Definately put something in writting, even if it is only between the 3 of you, it makes sure you all know where you stand and then you will be comfortable when telling him if he's overstepped the mark.
Consider questions like -
can he bring people back to the house? friends, long term girlfriend, sucsession of 1 night stands.
Housework rules - are you all going to just wash up your own stuff? will you take it in turns to cook meals for each other? Will you cook for your family and the roomie sort himself out?
whos going to clean communal areas like bathrooms, and how often?
Are you going to have communal food ect. where you all put money in a housekeeping pot or are you going to give him a cupboard in the kitchen and a shelf in the fridge?
Are you comfortable with him wandering around in a towel out of the bath or would you rather he dressed before he left the room?
Do you want him to treat the place like a home where he is as comfortable as you guys or do you want to remember that this is your house and he is just staying in it?
Do you need to set a limit on the noise if he is into his music?
There is a lot to sort out, but it can work really well, well, sharing a house can, I've not done it with a LO at the same time though. The important thing is to really know where everyone stands.