Replace the post. Fill in the hole with concrete after you remove the rotted wood. Get a post standoff that keeps the wood away from the concrete and put it in the hole when you fill it with concrete. Make sure it's exactly where you want it because the new post needs to be directly under where it is now.
Tell your brother that you never bury deck posts on concrete, always use standoffs.
Ron
Tell your brother that you never bury deck posts on concrete, always use standoffs.
Ron