Hi, A must is that you buy a 1 day travel card (after9am) for I think around 7 pounds and 1 pound (ish) extra for your son, it gives you until around 2am next morning unlimited travel on all tubes (underground) and London buses (RED).

Lunch at the rainforrest cafe cannot be missed as it has animated animals but do be aware that some do make frieghtening noises at random and my scare timid kids.
www.therainforestcafe.co.uk

Changing of the guard at Buckingham Palace at 11am everyday is also not to be missed as is the natural history museum (and it's free entry) and is full of interacting stuff for your son (min of 4hours recommened)

Check out visit London.com for loads of stuff and discounts.

http://www.visitlondon.com/offers/

Another great one is Martin Lewis the money man for upto 50% off vouchers in shops and restaurants.
http://www.moneysavingexpert.com/shopping/ and just click / print then take voucher with you.

HOPE THIS HELPS, I'M AUSTRALIAN AND ALWAYS LOOKING TO SAVE FOR TREATS. WAS IN UK FOR 20 YEARS, any other Q's please feel free to ask!!!
Regards, John.