As far as I am aware, when you go to OASC, you will be considered for your first career choice. If for any reason that is not available (because, for example, your aptitude results are not good enough), then you will be considered for the next one, and so on.
When I went through, those who did not pass certain aptitude tests were offered the option of changing their preferences. So those that failed pilot were allowed to add navigator, air traffic, fighter controller, regiment etc etc as they saw fit.
The number of people who are in the RAF who wanted to do something else initially is legion. My advice would be for him to put down what he wants, and then if he doesn't pass the aptitude tests for that branch, he can always have his application considered for something else. So if he really wants pilot or regiment, then put both down, in his order of preference.
Just a word of advice: if he puts down a branch on his application form he'll be expected to know all about it. So don't put down pilot, nav, atc, fighter controller, engineer, int, regiment, if all he wants to do is regiment or pilot.
SBW