I have a friend who has asked me and my family to move with him to his home, he has special needs and he needs our help. Is there any legal guaranty that after we move in, if he changes his mind for any reason, he can't just throw us out?

I would ask an attorney about that. Usually you can call one and they will give you some information over the phone. They will be able to tell you how involved something like that can be, but I would at least talk to an attorney before I made any type of decision.