I haven't done that one. However I have been involved with the Guildford Cathedral one, and as no one else answered. I assume they most run on similar lines. So it gives the boys/children an insight into what a chorister does, they play some games, learn some music, and sing a service at the end. At Guildford there is also a programme of talks for the parents. The aim is to talent spot, but also to give real insight so both parents and children have an idea of the commitment and if it is something they can commit to. They may also provide some information on less commitment heavy local choirs. It should be fun for him.