If you don't have a problem with the flashing then he could retrospectively construct some sort of goal post arrangement to ensure his roof is not supported by your wall. I'd want the drain sorted too, I wonder how he's built over that?
What does the front look like - does his garage have its own wall on your side?
You could write to him to telling him, that you've taken advice and believe his building is trespassing, noting also that he did not serve any applicable party wall notice either, and you intend to seek an injuction to have it demolished. You could suggest you want to avoid that and would prefer to have a remedy that means he can keep his building without it being supported by your property. Mark it without prejudice and see where it goes. It does no harm to attempt resolution.
If you get to an amicable agreement make sure you have a right to access for maintenance in the event that you grant him consent