What should I do? Should I tell him?

Long story short my sister is sitting in jail for bad checks and child support. She has been lying to my entire family and to her boyfriend of 6 years, whom she had a baby with last year and subsequently gave up for adoption. I have been speaking with him the last couple days regarding trying to get her out of jail and not once has the child come up in conversation. I find it odd since if he knew he'd know I know. I am the only one in our family she told. Why would he not bring it up to me? Maybe because she never told him? She didn't find out she was pregnant until she was 6 months along and she gave birth alone and the adoptive parents took the child straight home from the hospital. If he doesn't and I tell him it'll crush him, even though he never wanted kids. Do I tell him I know?