he needs to register for online services
if he has registered, on his normal reporting date, one of the options in his online services will be "report employment income" or "report income and participation" on the home page (the wording differs depending on circumstances
he selects the hyperlink, answers the questions, hits submit - done
just make sure that at the end it says it was successful. if some questions are answered a certain way, it will be unsuccessful and he will need to ring to discuss what has happened and have the payment finalised there

the other option is to register for online services (telephone) and you can do it that way - the computer asks questions, he answers.
you can have speaker verification (which, personally, i HATE) or can just say no when prompted for that, and set up a 6 digit pin