How to confront a guy without scaring him off?

Anonymous
This guy knows he did something wrong. We've been on and off, and he knows I like him even though we hadn't talked I na few weeks. We hooked up last night, and he didn't tell me he was seeing someone, I found out whenever she called first thing in the morning to bitch at him for something (not sure if they're official or not).

I need to confront him for my own sake, but I'm trying to figure out how to go about it. I need him to be receptive to what I'm saying to obtain the answers/explanation. Generally, I'd choose not to contact him and act as if I don't care and let him come to me, but I'm not sure that he will in this case. And I know I'm going to obsess until I talk to him about it.

All of this happened last night/this morning. Would it be a good idea to confront him about this tonight, or should I wait? Also, how do I go about asking him why he didn't tell me he had a girlfriend without coming off like I'm attacking him? He and I are alike where we avoid conflict, so I know that if he feels attacked, he's going to do what he can to avoid discussing the situation.
How to confront a guy without scaring him off?
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