Whether he is at school or applying as an individual, your son will apply online. As he left only last year, it seems likely his school will want to write his reference and use their Apply system to do this. He should talk it over with his school.
He can choose whether his application is linked to the school, so that they check it and send the completed application to us, or whether the school provides a reference only and he manages the application himself. Once he has registered to use Apply, he can select his preferred option in the 'Options' section. The school will give him a buzzword to enter.