He believes Will. He believes in Will's skill at Seeing People. And it's always easier to see others than it is yourself.

Knowing for a fact what composes him. He... doesn't know what that will be like. What it will look like. He remembers asking Dani what if he makes me who I am? and he was really asking but she didn't have the tools to know the answer to that.

Ultimately, what Will is offering him are the tools to find out himself.

"Okay... how... do we do it? The Enclosure?"