As a single mom I have to say - EWWWW. So unacceptable.
Revolving door policies on boyfriends are just confusing to children. My son never met anyone I dated after about age 3 - before that he was just too young to care. And I never had one sleep over, much less share a bed with my son and a "date". Ewwww, again.
I don't know what the child custody/support papers in place say, but mine has a clause in it barring any persons from the opposite sex, not related by blood or marriage, from sleeping in the house when the minor child is present. Since my Ex had a revolving door policy with girlfriends, and my son, at about age 3, asked me "why all the other Mommies sleep with my Daddy" it was a welcome addition to my custody papers.
Your step son's mom is not obligated to introduce you to her boyfriends, but you should all definitely have a talk to her about exposing the child to a series of random men. And express how unacceptable it is to have him share a bed with those same random strangers. Oh, what a potential recipe for disaster.
Since the child obviously stays at both your home and her's, why can she not limit her sleep overs to the nights when he is not with her?
Oh, and don't get me started on having live in boyfriend's with children. I am just very old fashioned about what children should be exposed to. It is confusing enough for them when Mommy and Daddy don't live together - but to add boy/girlfriends, random dates, and live ins to the mix, is just creating a untenable situation that children just don't understand - and that causes confusion, fear, and insecurity.
I hope that you can help her understand that - for the child's sake.
Good Luck
God Bless