"He's been allowed to realise it too much."
Now why would a manager want to hide one of their people's value to the business from them.
Let me see...
To claim it as their own
To increase their own value by devaluing his.
To keep their salary down.
To make sure they don't get uppity and take your job.
Which one is you?
I'm really interested in how you put best face on this one.