When exactly can you trigger his ability? When he exhausts? Or after you've seen the shadow affect and damage is about to be applied?

You can trigger his ability after seeing the shadow card and before combat damage is dealt. It is an action and there is an available action window after resolving the shadow card.

For reference, there is an action window after each step of an enemy's attack in the rule book.

That's what I thought. So I can also heal him after taking the damage from the ability, but before enemy damage is applied. Just wanted to be sure. Thanks!

That's what I thought. So I can also heal him after taking the damage from the ability, but before enemy damage is applied. Just wanted to be sure. Thanks!

Somehow I never thought to heal him after he takes the damage from his ability. Nice!