NC is a good idea but the guy needs to understand that you don’t understand what they expected or that you might be willing to change behavior if it is talked out, if you are. Did he give specific examples about what upset him or was it just a general feeling of his? Are you willing to work that through with him?
If so, my 2 cents would be to continue no contact after sending him something explaining that you might have been doing things that upset him without knowing what it was and you are willing to talk it through and change your behavior so you can both be happy in the future, if that’s the case. This puts the ball in his court in terms of the relationship.
Guys can be difficult to understand sometimes when it comes to how they feel about other guys, especially when it comes to their girlfriend and interaction with other guys.