My son's mom gets free rent and has someone living with her illegally. What can I do?

My son's mom has 3 kids total and because she's an unemployed single mom, she gets free rent (I've never really understood this, but that's another question). Anyway, since early March, she has had her latest boyfriend living there with her in the apartment. He too is unemployed, and I found out that he's an ex felon with admitted serious issues with anger and agression. He apparently had a mental breakdown last fall. I am very concerned about him being around our son (who is 19 months old). My lawyer told me it is a felony for her boyfriend to be living with her since her rent is free, and I have reported it to the local housing authority, but every time they confront her, she tells them her boyfriend is "just visiting." In reality, he had been homeless when they met, and he has never left her apartment a single night since early March. My question is, how do I go about proving this, so I can get him or both of them thrown out of the apartment, or at least force them to pay rent? We are currently in the middle of a vicious custody battle, in which she is lying about every imaginable thing, and since no visitation has yet been established, she is not letting me see my son. I am very worried about his welfare, and am outraged that I scrimp and save to make my house payments, and she and this guy are pulling a fast one on the system. Any advice at all on what I can do to prove he lives there will help, or is there a higher authority I can take this to besides the housing authority? Thanks!!