Get a few books about London from the library. Perhaps a mixture of fiction and non-fiction. We also bought our DC the I-Spy London book.
Your Tower of London tickets allow re-entry on the same day (just ask for a free re-entry ticket as you leave). If I were you, I'd get there super early, do as much as you can until your DS gets tired, and then go back later in the day for anything else you missed or to see he Crown Jewels again!
There's a Tower of London app which you can download called Time Explorers which is especially for children. There are also a couple of family activity trails which you can print of the website. During school holidays, special events are usually available aimed at primary school level children, such as arts & crafts. The Crown Jewels is usually one of the busiest exhibits. To avoid the long queues, arrive when the Tower opens and visit the Jewels first.
Check the Tower Bridge website to find out what times it's due to lift up. It's quite a sight if you've never seen it before!
Coram's Fields playground near the British Museum is equally as good as the Princess Diana Memorial playground, if not better!
If you fancy a trip on a bus without paying for an open top bus tour, the RV1 bus from Tower Gateway to Covent Garden goes past many of the big sites taking about 30 minutes. It’s a great alternative if you think your DS would like a trip on a bus but you don’t think they will sit still through a long tour.
The Museum of London Docklands is quite easy for you to get to and the Mudlarks children's gallery is perfect for little boys! It's free but you need a ticket. You can book one on their website www.museumoflondon.org.uk/museum-london-docklands/permanent-galleries/mudlarks-childrens-gallery My DS also loved the London Transport Museum in Covent Garden. It's fairly pricey as museums go, but it's so interactive and there's lots to climb on and vehicles to pretend to drive. Plus, the new Future Engineers area for families and children looks amazing.