What are the most appropriate approaches to rebound and cope a traumatized employee mentality after exposure to a Critical Incident?

First you have to consider not to drown him/her (Drowning Therapy; to through someone whose afraid of swimming all the sudden in the pool as an example); don't push him, and give him all the time he can have. Be patient with him and he will recover. Some people amaze you with their strength; as much as they are traumatized, and we all might think they need a long time to recover, you find them heroes.

Try to avoid anything reminds him with his trauma. Start with simple tasks he used to do, and you can use what he loves to do.

Don't allow everyone to treat him as a walking dead; keep on staring at him, treat him as a baby or talk behind his back. Such attitudes would affect him badly.

Have someone close to him, to be close to him. Let him be always there for this traumatic person, any relapse he might have; with this person around; he would pass it easily, and it would be useful for him to get better.

Try not to focus on his mistakes; tell him that we excuse you; but you did a mistake (point at his mistakes; you don't want him to get used to it, and after recover, he might keep in mind; "No one would blame me!"). But, don't be rough on him.

Consult a therapist or a counselor; to be sure that you are taking the right steps. And he will help you know when to push him.

Encourage him whenever he do right, or pass a task effectively.

That was a general way of treatment; but each traumatic has his own way of treatment, this way might work for most of them. But some might have their own way, for example; pointing at his mistakes might be bad for him.