My boyfriend has syphilis?

My boyfriend just told me he has syphilis. And now I'm at risk. I thought how was that possible? He told me I was the only person he had with this year. It's when I pressed on it that he told me he had with another guy. I thought we were monogamous... he lied to me all these time. Now I'm worried about contracting other STDs from him as well, like HIV. But there's a 3 month window period so I don't know for sure. He asked me if we can start a fresh. I don't know what to do. He lied and he cheated. But I grew used to him. Syphilis is curable so I'm not worrying too much about that. The bigger question is HIV. What if we both got it? Should I stick with him then? It's a heartache because I invested so much time with him. I got used to him. I need some advise. Please help.