If you have a ticket, then he doesn't need one for the tube.
(The generally used method seems to be to use the wide 'luggage' gates, and just go through together, rather than asking to be let through by the staff.)
I do not know if there is an easy way to determine which National Rail services (other than LO) are valid for free travel for children up to 10 years old.
Ah good, that's straightforward then, it's all the routes that are priced at TfL rates, I wonder why they don't just say that and link to the map.It's the green routes on http://www.tfl.gov.uk/assets/downloads/tickets/national-rail-map.pdf
It's the green routes on http://www.tfl.gov.uk/assets/downloads/tickets/national-rail-map.pdf