My former flatmate was structural engineer so take this answer as a second hand guesswork from my conversations with him. Because I don't know anyone else in that profession I assume he was typical example.
Structural engineering was his Master while his undergraduate was Civil Engineering. So your hero knows a lot about building things. He also knows a lot about building materials, and though most of the modern materials are not available to him, he knows very well the classical ones too like brick, stone, timber, mortar and their properties.
Your hero also knows how to use iron, I expect this to be quite expensive in the 13th century but maybe its use on few strategic places won't break the budget. He also knows about roman concrete if volcanic sand is available. Though Portland cement with reinforced steel bars is not available, I assume he could think of poor man substitute with Iron and whatever materials he has at hand.
His Math is very strong, and being able to design and calculate the strength of each part of the building, would be of much use to Master Mason who probably works only through experience. it would be tedious without calculator but it could be done.
Also he certainly knows about many famous buildings through the history including medieval ones, I expect that 7 century advantage vastly exceeds whatever special knowledge Master Mason has.
In short I expect him to be very, very useful, working together they could build a hell of a castle.