Dots,
At that age it's hard to lay down the law. My son is 24 and almost out of the house now. He's got a place that needs a bit of work. When he returned home about a year ago (after a year in his own appartment) to do the same thing as your son, save for his own place, I didn't give him any curfew but like Mountain Ash said, a phone call is always appreciated. He was always good about telling me where he was going to be and he still is. I asked him to let me know if he would be home for dinner and most nights he's good about that too.
He's responsible for his own laundry and bills and it's been working out very nicely here.
I guess my suggestion is just to sit down with him and explain that you need to know where he is and when he'll return only because it worries you when you don't know, not because he's a baby who needs his parents looking out for him. I find that if I know in advance that he will be late, I can sleep better. It does take some getting used to though. I used to get up in the middle of the night to check if he were home but now I am sleeping right through.
Good luck!