Std drama

So my bd and I had casual sex . He called me 2 days later apologizing saying he has a gf and feels horrible for cheating and he’s never cheated on her before . I had no idea he had a gf and was very upset with him . I was feeling a bit off and went and got tested and found out I have an std I haven’t had sex with anyone since I got tested last time and came out clean so I know I got it from him . I’m going to tell him today but I know that’s about to ruin there relationship and I feel bad.