There are many, many things to do and to walk around the West End and City is probably one of the best. You'll see things that lots of people in London miss by being on the Tube, never mind tourists, and will definitely have a great time.
Good suggestions above. Oyster card will be cheaper for you but Travelcard for your son unless you want to arrange a photocard Oyster for him (
www.tfl.gov.uk).
St. Paul's cathedral is amazing, the Natural History Museum and Science museum are both great fun and very educational for children (I love going still - there's plenty for adults too...) and really, you can't go wrong.
Here are a few sites you might be interested in:
http://www.visitlondon.com/
http://www.viewlondon.co.uk/default.aspx
http://www.allinlondon.co.uk/
http://www.londontourist.org/museums.html
and last but not least (sorry, I just love doing it):
http://tinyurl.com/bbptft
Enjoy yourself!
