As a parent, you'll have to accept that your child is an adult and can make his own decisions, and your child will have to realize that living in your house might entail more rules than he is used to.
House rules could be related to having house guests or parties, and your stance on smoking and drinking alcohol. Just remember, although your child should be expected to abide by the rules of your house, they should be age-appropriate. For example, setting a curfew for your child might not be realistic, but expecting him to call if he isn't coming home for the night is.
However, charging your child room and board, no matter how little, can help him learn to manage his finances and prepare him to live independently when he moves out. It might also reduce the chances of you becoming resentful if he is spending money elsewhere while depending on you.