I am sorry to hear that you have experienced this.
I have little in the way of advice regarding relationships, but my guess would be that it is best to reconsider your standing with your "boyfriend".
If you really wanted, you could confront him about it, and see what he has to say on the matter, if you are willing to go through the trouble of doing so.
Was the "messing around" serious, or was it simply an interaction with another person which might be misinterpreted as something serious?