IF he really, really likes superheroes you could take him to Tussauds (go fast track for your sanity) to have selfies with the superheroes. Likewise if he loves rides, take him to Winter Wonderland (take lots of cash to buy tokens, the rides are expensive)
Or go down another route entirely - Wonderlab at Science Museum (paid entry) has just opened and is meant to be fab (am taking my 3, 7 and 10 year old this weekend - they cant wait). Also iceskating (if weather is nice) is a good call. If weather is crappy, bowling and pizza always a good option with my 7 year old.