According to this page,
Eastbourne Buses Limited - Fares & Tickets - Fares & Zones
a 'child' is charged 70p for bus fare. However, they don't define what age constitutes a 'child'. Perhaps extremely young children and infants are allowed to ride free....the fares page doesn't say.
I doubt that he had a pass as the mother seems to indicate that the boy is not used to travelling on buses alone.
A question also arises as to why did the bus driver allow such a young boy on the bus unescorted, without at least asking a few questions of him. Perhaps the boy got on the bus with other passengers and the driver assumed that he was being looked after.
Unfortunately, the article does not go into this level of detail and we can only guess.