You did the right thing and did not screw up. Have a calm, quiet talk with your son about how it's your responsibility to see that he's safe. If you haven't had "the talk" yet, now is the time to do so.
Let him know that you understand that he wouldn't want to tell you anything again but you hope that he realizes that not talking with you is not helpful to him. Talk about the complicated decisions ahead of him (not just in relationships) and how you rely on talking with others to sort out good alternatives. Emphasize for him how much you love him and how important it is for him to have reliable adult guidance as he matures. Perhaps read up on and talk with him about the way in which our body and brain matures. There are some good books on this subject that you can buy or check out from the library.
Then I'd get him interested in some high energy activities, such as playing games at a rec center. Perhaps he already has some interests you could encourage so that he has less time to focus on a girl. Enlarge his social experience by getting him involved in group activities. Church groups, if you're religious at all are good for keeping kids occupied and learning. He could join a Boys' Club, Cub Scouts, or other boy's club. He's at an age that is usually more interested in boys' activities than girls.
I might make an appointment with a child psychologist for an evaluation. My granddaughter is 10 and the boys are still being goofy to get girls attentions. And my granddaughter and her friends still think kissing is gross. This seems a bit young for this serious of kissing.
I also suggest that it's possible that your son told you about it because he wanted to be stopped. I'm guessing the girl is more mature than he is and even tho a part of him likes it he also knows that this is not appropriate.
My daughter would tell me she wasn't telling me anything again but she always did. What I found to be important is to rarely agree that I wouldn't talk with someone else. I told her, during a couple of general conversations and not when she was confiding, that as her mother I would sometimes need to talk with another adult about what she had said. She still confided in me. It helps when we are honest with our child so that they don't expect that their confidence will automatically be kept. I did agree to not tell during specific conversations. And sometimes I felt I needed to go back on that agreement and so I would tell her before I actually talked with anyone else.
Sometimes, I would give her a choice of talking with the other person or I would do so. For example; in this situation you could tell him and the girl that you felt it was necessary to talk with the girl's mother. Did they want to talk with her before you did.
Later: I know that kissing goes on in elementary school. In the specific instances I've been told about the kids watched movies and TV programs that included sexual behavior. I suggest you monitor what he's watching and see if he's trying out what he's seen. Many TV shows that are geared to teens seem innocent and often younger kids watch them causing them to think kissing is appropriate.