Well done to your DS, he must be very happy getting the school he wanted.
You have probably already seen Google Maps travel planner, but in case you have not, it does have a very useful tool for trying out various transport options, train-bus combinations etc. plus it allows for real-time traffic variations if you consider driving in.
I agree with ToadMum, though, re. driving to school yourself - that would be quite a journey there and back, twice a day, and unlikely to be much quicker than a train. Your DS will have to be at school by 8.15 or 8.20am at the latest, so you would be stuck in the worst possible rush hour heading into London... Unless you have to do that journey each morning yourself anyway, a train is probably a better bet.https://www.google.co.uk/maps/dir/Horle ... ol+(Stop+K