A little trick,
If he puts down more than one branch it is more to learn for the interview as they should question on all branch choices.
However, if he does make it through to past the interview, you have an 'admin' interview and at this point you can add extra branches, without actually having to chat about them at interview level - that is the point to stick down any extra if you really desire.
There are many pages of advice for Filter/OASC on the Armed Forces section of
The Student Room if your son is interested.
All the best
