Hmmm, are you certain he understands what he did wrong and why it was wrong and what would have been a better way of handling it?

I used the word "unacceptable" many times on my DD over the years before she finally broke down once and explained to me how much she hated that word. I learned that most of the time she really didn't know why her transgression was wrong and she just began to internalize that we found her to be unacceptable :-(.

I think Grinity's advice above is excellent.