Ex is dating without any concern for our son

I have no problem with my ex dating. My problem is that she has been ignoring my concerns about bringing my son along. I moved out of her house 3 months ago and visit often to see my son, seeing as how i live only a stones throw away. I have repeatedly expressed that I wasn’t comfortable having someone that i don’t know being around my little man so much until i got to know him better. Well, over the weekend i stopped in like i normally do. Only to discover that she not only had this boyfriend stay the night in her bed with my son in the house. But told me that the progression of her relationship was none of my business!
From what I’ve heard from others, this is apparently a pretty big offense and i can have my son removed from her custody. This is of course not something i want. But i was wondering what kind of trouble legally she might have gotten herself into. And if shes not willing to listen to me, what can i do to protect my son from any potential harm.
I have been more then calm and reasonable in my requests, but if my requests are constantly ignored then how can i turn them into legally binding demands?
We are separated, but no paper work has been filled yet. I guess im just wondering the best course of action to protect my son.

*not an attorney. You don’t really get pick or approve of who your ex dates after date of separation. If you know his name its probably a good idea to run a criminal background check and/or sex offender check on him. Its probably best that she avoid stuff like this, but its only an issue if you you two are separated but trying to reconcile or if her current actions collaborate her actions before the date of separation.

Unless the person is dangerous to the children the court is unlikely to do more than order no overnight guests.