If he wants to go sightseeing somewhere, he's going to have to plan it properly. Create an itinerary. Create a schedule for calling. Figure out what sights he plans to see. Even research the bars he wants to visit. He has to make you some promises and show you he can be responsible.
Review with him some basic safety tips, like not going off in a stranger's car, guarding your drink, etc.
I think this could be a decent "practice run" before heading off to college on his own.
(My 15yo is jealous, even with all the riots on the news, she'd LOVE to study in London! She fell in love with the city when she visited last year.)