Frankly, either trip means a lot of time in the car. Does your son like riding in the car on very narrow roads?
I took my son to Scotland at that age, when I was working on The Renaissance Club. We played at North Berwick, climbed the North Berwick Law, drove out to Tantallon and down to Bamburgh Castle and Holy Isle. We went into Edinburgh. It was a great trip, with plenty of adventure, but also more time to chill. Just a thought.