I enjoyed seeing Stonehenge actually. So many people told me that it was overrated etc that I really wasn't expecting much. I just read up a bit on Stonehenge and various theories about it through the ages, and also included a nice wander through Salisbury and, more particularly, the Cathedral. I thoroughly enjoyed it.
Are you willing to rent a car? If so, consider doing a mini side trip to include Bath, Bristol and Wales.
Hop on hop off tours (as suggested by Scaryant) are a great way to see the sights. At the very least you can see where things are and where they are in relation to each other. The tour can be a bit overpriced but they are easy and do tick off a lot of the sites you may want to just get a picture of.
Go on a London Walk - traditional ones like the Jack the Ripper tour would be great. But there are lots of tours. They last about an hour or so - so it's not like you have to plan your trip around them. Just turn up on a day which suits.
If your son likes movies, find a guide to popular movie locations in London. There are (obviously) a lot, but if he likes a particular movie it could be fun.
Take him to a pub for a meal.
Imperial War Museum is great. Do war stuff.
Do touristy stuff. Take him to Harrods and get your son to try on a £20,000 Armani Leather Jacket. Actually, if he is 17 he would be mortified at doing such a thing. So I would definitely suggest doing that!
I probably can't help more than that. I love historic sites and did as a teenager. I imagine that would bore a lot of teens.