Do I tell bio mom or no?

My step son came up to me today and said he really needed someone to talk to and he's freaking out. I said sure and he told me to promise not to make fun if him. I said I won't. He told me he thinks he has an STD. He told me not to tell his dad, which is understandable, his dad will murder him, but he also says his mom doesn't know and he doesn't want her to know either. I don't know his relationship with his mom. Would it be wrong to not tell her or my husband. He asked if I could drive him to the health department tomorrow and I said sure. He will 18 in a few months so I think I should respect what he wants. Would I be wrong to drive him there and not tell anyone?

I know it seems wrong to go behind their back, but his dad will actual put his hands on him if he finds out. Like I said I don't know how things are with his mom, but I don't want him to be put in an unsafe situation.