Tough one. His fear is understandable.
The mind is normally the last to heal, it has to re-learn to trust the knee. The knee "betrayed" him badly so re-building trust is not easy. The minds number one goal is survival, and it is easier to survive if you aren't injured.
So talking about that with him for one. Let him know it is understandable. Let him be OK favoring it.
Strongly encourage him to visualize himself playing full out on his knee. He needs to see it in his mind AND feel it in his body while being safe (visualize it at home or in the dugout). Over time this will help rebuild the trust.
p.s. I don't mean to just advertisize my product, but it will guide him through visualizations -- easier to be guided through them than do them on their own.