If your son's friend always ask to spend the night at your house...

He must feel very comfortable at your place. There could be a thousand reasons why he doesn't ask for your son to stay at his place, but at this point we would be guessing. I would continue to let him stay as long as he is behaved and you don't have any true complaints about it. He may need a safe place to be more than you know, again these are guesses but if his parents have never wondered about it then who knows. We have 5 children and have found that although our childrens friends love staying here especially those that are the only child but our kids like to stay where there is less interference sometimes. Maybe it is something simple but I can't really think of a right way to ask him that question. And if it is something unsafe you don't want your son there anyway. There is never a fair thing for parents when it comes to this, I can understand where your coming from and would like to maybe have a night without children yourself. As a mother with 5 children this could be another thought both my sons have friends that are only kids and on the wild side (I hate having them over) and then they also have those that are very polite and I can truly enjoy. But always when there is an extra mouth to feed or another child to tend to period it becomes a chore. So I guess I kinda went on and on here but the point I am really trying to make is you may never know but don't punish the child for this take it as a compliment to keeping a nice welcoming home.

Odds are he is ashamed of his family or they won't let anyone stay over. I wouldn't do anything, you may be saving that kid from a bad situation.

Have you asked your son or his friend why he does this? He might have a good reason. Maybe he has problems at home? Arguing with parents? Small child crying all night? And he just wants to get away from it and since he wants to get away from it assumes that no one else wants to be brought into that environment for a night. Maybe his parents don't allow other people to sleep round? Therefore it would be a little unfair to stop the child sleeping round, when it is the parents fault that it doesn't work both ways.