Notting Hill is a waste of time although Portobello Rd market is OK on Saturday mornings... It's all a very long walk from the tube though...
Ever seen Harry Enfield's "I saw you coming" sketches? They're set in Notting Hill.
Carnaby St is a short street with tourist shops - not much point.
Covent Garden is good fun - lots of buskers all day doing everything from close up magic to tumbling and the "escape from a straight jacket" thing. Lots of little alleys to wander in and out of, lots of over priced shops ! It's also got the London Transport museum which is very good and (I think) you get a free return within a year ticket (?)
HMS Belfast might interest an 11 yo boy but it'll cost you about twenty five quid - same with the Cutty Sark in Greenwich.
The buildings at Kew Gardens are amazing but I wouldn't say a great day out for 11 yo - it's a botanical collection essentially and way out West on the District line.
As mentioned Kensington is a good bet and Barkers has a HUGE roof garden (which is not always open to the public so worth checking before the off).
The Boat Race is on This weekend - could be fun to watch from a bridge (pub).
If you take the Jubilee line to North Greenwich you can jump on the "Air Line" which is a very high cable car across a wide stretch of the Thames - amazing views and quite an experience.
Crystal Palace has 30 full sized Dinosaurs placed around the park. Weird but different...
For my money the Golden Hind is one of the best kept secrets in central London. £5 adult / £3 child. It's Francis Drake's real life actual pirate ship so some real history and you've snuck in some educational value too. Right next door to the Clink Prison museum (which is great and again, quite reasonably priced).
There's the Imperial War museum of course which is housed in what used to be Bethlam Royal hospital ("Bedlam"). It's all guns and tanks and planes so you might not love it but he will !
If you look up the times you can sit outside the Tower of London on the river and watch Tower Bridge open and close to let tall boats through. Kept my lot entertained for hours.
LINK: www.towerbridge.org.uk/lift-times/
You can actually visit the bridge too and walk across the top but I've never done it so couldn't comment on entertainment value.
One last thing - if you write to your MP they will (must actually) arrange for you to climb up Queen Elizabeth tower (what everyone calls Big Ben). This is really worth doing but it can take weeks / months to arrange in advance. (Summer holidays are coming...)
You can also request lunch / tea on the terrace of the house of commons which is just cool...
Have fun !
PS:
Avoid the London Dungeon. Overpriced and will give 11yo nightmares for weeks.