Actually, I'm in favor of being brutally honest with him.
But here's the catch to my support:
1. You can't expect him to change his mind and come back.
2. You have to be prepared that he won't care and not let him persuade you that you're wrong.
I say this because, if you can follow those two emotional rules when being honest with him, you can get the closure you sometimes need. You can walk away knowing that you voiced how you feel and then can successfully let go of that situation as a whole, making yourself a little more ready to move on.
But this can also backfire if you don't follow those two emotional rules. If you let him persuade you that you're wrong, like I have done in the past with some guys, it'll make you feel worse. And if you care what he thinks, then you're doomed from the start because you won't really be brutally honest. You'll just sugar coat it and hope he understands. And if you think to yourself maybe he'll come back after it, after thinking long and hard about how he did you wrong, you're also doomed because that does not put an end to the story. Instead, your heart feels like it's not over.
In short: It's tricky, but it can give you closure. You just have to remember that it's over entirely.