Having been through the system a few years ago, the interview is a chance for the recruiting officer to get to know your son a bit more, as well as making sure he is up to date on current affairs, and some basic military knowledge. He will not expect that your son be fluent in tactical doctrine, only that he has a basic idea of the Army's main structure and important issues.
With regards to your going along, I would suggest that you go along with him to the interview, but you will probably not be allowed into the interview room. By going along you will be able to ask the interviewer any questions you may have.
If your son is having an interview for a University Bursary, then the Army will give him some money as a lump sum, but it is not enough to cover all his living expenses. The OTC can be a way to offset some of the cost, but he will either have to get a job, or you may have to step up.
Hope this helps
dep