Crouchendmumoftwo I've had similar happen but mainly when I was very much younger (and less confident), and sufficiently infrequently to not consider it to be a given of Tube travel. It's not even something I considered as one of the 'risks' when first reading this thread. I know of DC who've been inappropriately touched on the bus, or flashed at en route home (walking), so it's not unique to the Underground.
OP I would consider doing more than one recce of the entire area around the Tube stop where you're DC will be getting on/off just to familiarise them with alternative transport options. Children need to know how to navigate themselves (on foot, if need be) without getting into a panic, in the event of their normal route/mode of transport not being available. I think this is vital for their peace of mind and for yours too!
Hopefully your DC will make friends quickly and find that he has travel 'buddies' for at least part of his journey. That makes a difference although it brings with it different issues (waiting for friends and ending up being late for school being the main one!).
Issues of transport and independence seem like huge mountains to climb for all but a very few 11 years olds starting at secondary school. However, usually within a term or so they're confidently navigating journeys here, there and everywhere.
To my mind it is all part of being a parent to educate children to use public transport safely, independently and confidently.
Good luck to your DC, OP.