How would he do that?
And
How will he do that?
How will he do X?"
He wants to do X and is asking how, or
He wants to do X and we are trying to guess how it will happen.
How would he do X?
Same as above, but there's an if Y implied, or
Same as above, but in the past; i.e. he wanted to do X and was asking how, or he wanted to do X and we were trying to guess how it would happen.