well, i have never raised an older child (mine is only 17 months!!) but only 7 years ago i was 18 and living at home... so i guess this is what i remember.
he is an adult, and he can certainly make his own choices and live his own life, however, if he is living under your roof, there are still going to be house rules, blah blah, and just simple things out of respect that he needs to do for you. he should have to pitch in somehow. whether you decide to charge him a little rent, or he buys supper one night of the week, or he does the garbage/dishes/other chorse... something to help the household. adults have responsibilities, something moms know well.
also, he doesnt have to give you every tiny detail, but he should tell you whereabouts he is going and when he should be back. you are his mom and no matter what, you will sit up and worry if you dont know where he is right? i know i would LOL. so out of respect he should let you know what time he will come crawling in. that will also let you know that its him and not a burgler (spelling bad) or something like that.
i would keep the house rules the same, but give him the option of being able to present his feelings about such and such a rule, and consider his opinion. you (and your husband?) might decide that one rule can be bent since he is an adult, another might not because its a general rule of politeness... you know???
i hope that helps. like i said, i havent yet done it, but those were the things i remember from being that age.