My ex-husband's girlfriend and 3 daughters are spending the night at his house?

Since the house is kind of small, the three daughters (ages 4, 10 and 12) are sleeping with my son (13) in his room several nights a week. He has a bunk bed with a twin mattress on top and full size on bottom. He has not hit puberty yet, by the way...however I am VERY bothered by this sleeping arrangement. I think it's innappropriate...and my son complains that sometimes the girls bicker and keep him up late. I have told my ex-husband that I am against this, but he avoids the subject. Typical conflict-avoider, that he is...but I feel very strongly about this. How in the world do I get this to stop? Any suggestions?