C.H.
What a great opportunity for a wonderful life lesson! Life is difficult sometime, this will be one of those times for you, your family and theirs. But aren't you lucky to have someone share a roof over your heads in these tough times. There will be other trying times your child will be exposed to but this one will be great practice because you all have to get through it together.
The role modeling you do will be important - every house has their own rules and accepting these rules is what you all have to do in exchange for their generosity. Your son will survive. He will learn great lessons in empathy, gratitude, sharing, social skills, conflict resolution, problem solving, all things everyone has to learn in life. Just be matter of fact with your son about how different things will be, validate his and everyone else's feelings about it and prepare the best that you can.
The worst thing you can do is coddle him and try to keep things the same because they won't be. Remember, change and acceptance is good - it often produces wonderful growth if you are looking for it and open to it. Good luck!