As PP have said I think you need to plan exactly where and when you are going to be to drop him and then pick him up, as those are going to be non-trivial on Sunday. Are you driving, when you say "drop him off" and "pick him up"? Are you dropping him at a train/tube station?
He knows he needs to come in a day or so before to pick up his race pack, right? Or have someone else pick it up. Saturday there is a train strike....
Where are you dropping him? You won't get close by car, and at the moment he won't know what colour start area he is in I would imagine so which tube/train station he needs won't be clear until he gets his pack I don't think.
Is he then coming back to Greenwich on the tube for you to collect him there? Or are you going to try to park somewhere nearer the end (which will be a serious challenge). If he's getting the tube back, it's quite likely to be 2-3 in the afternoon at least - he isn't guaranteed to start at 9.30 (his start wave will be in his pack), so he might not actually start until 11.
Either way, the roads and tubes will be very busy.
If you want to drop him and then drive into town, I would seriously consider booking a car park space somewhere like Hyde Park, although the drive will be horrendous thanks to the road closures.
Don't you want to watch at any point?