Definately do the Science Museum, my hubby is a big kid, and he loves it there,,lol. It makes me think of EPCOT when we go there, lots of hands on things for him to do and expirence.
Also, gotta echo Hamley's,, nearly lost my hubby in there,, did I mention that he's a big kid?
Take him to see 'The Lion King', or my personal fav, "The Reduced Shakespeare Company's, The Complete Works of Shakespeare",, very funny,, and they love seeing kids in the audience to play off of
My younger daughter has their dvd, and makes me watch it with her alot,, it totally cracks her up
One thing you can do to get around easier, is to buy a ticket on one of the tour bus circuits, and hop on and off whereever you want to
The ticket is a 24 hour thing, altho the last bus is in the late afternoon. Have done this a couple of times, it passes Hamley's, the Science Museum, and lots of other places you may not have thought about,, also some of them give you a ticket for a free boat ride from the Tower of London to whereever it stops,, I forget,,LOL.