Need advice

My boyfriend and I were going through a rough patch, and during this time he slept with another woman twice. This resulted in the girl being pregnant (supposedly by him). Today I learned the girl has an STI and now my boyfriend is seeking treatment too. All of this started from a little hook up. Should I stay (that’s what I want to do), or should I leave?

P.S. - I do not have the STI & I don’t think it’s his baby.