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It may not be a good idea to bring it up right now because you just started having a good relationship with him after what happened with the two of you. Talking about it now may push things back where they were, are you ready for that possibly happening? If the answer to that is no then I would work on the friendship a bit longer and then after a few months when you feel that you may possibly be able to bring it then do so but not right now it's important to be able to talk to him about it and keep the friendship that you had. There could be many reason he stopped talking to you after that, he may have been embarrassed because he used a friend in that way, he may have had feelings for you but found it awkward after you slept together, he may also have needed someone to be with and choose you and then realize what he did and wasn't quite sure how to talk to you after that happened to be honest a little awkwardness after a one night stand or sleeping with a friend happens sometimes and it's always important to communicate your feelings about what happened but right now just isn't the time give it at least two months to work on the friendship and then a way you can bring it up is to apologize for what happened or the way it happened and then it will open up the lines of communication and he may be able to tell you his side of what happened and you will have a clearer understanding of his thinking then.