If my 17 year old son’s mother is allowing him to have his girlfriend stay the night and I considerate this completely inappropriate, legally is there anything I can do about it?

If there is a custody order in place that sets out the parameters for custody, then you could file a motion to modify those orders.  Courts do sometimes place restrictions on parents and do not allow their paramours over while they have possession of the child.  Because of disturbing trends like the one you are facing, some courts have had to impose the same type of restrictions for the girlfriend/boyfriends of children.

However, the problem you will be facing is your son's age.  He is not far away from 18 and he has already reached the age of consent-- so you might be getting an order which will soon be moot.  As a back-up plan, considering finding out a bit more about the girlfriend and having a frank discussion with your son about the law.  If she is under the age of 17 years old (i.e. 16, 15, 14...), then he has technically committed the offense of statutory rape.  If she is within three years of his age and the sexual relations are consensual-- then he would have a defense.  But if she's more than three years younger-- he won't have a defense and could be required to register as a sex offender for the rest of his life.  Even if he is just arrested for the offense, the arrest for a sexual offense will haunt and limit his career options.  He also needs to keep in mind that the key to this defense is "consensual."  If they ever have a "falling out" all she has to do is say that she refused consent-- and if a jury falls for it-- he'll become a convicted sex offender.  Seventeen year-old boys are rarely thinking about these consequences when their hormones are running rampant-- but if he insists on continuing with this course of action, you might want to tell both of them to start building up a legal fund just in case..... because she is allowing her home to be used for the activites, she could potentially face some type of criminal charges (failure to report sexual abuse, injury to a child, or even as a party to the sexual assault by encouraging and facilitating the activities).  Being called before a grand jury is a hormone sobering experience he should really try to avoid.

The next obvious discussion is what happens if/when a baby comes along.  This is a serious issue which could affect your son's life for a long time.  It will be frustrating to do a modification of the custody orders or to talk to him about the range of consequences, but considering the potential devastation of these consequences it really is worth at least putting for the effort.  Good luck!