@ Greengrass
Applied to do Maths this year, I chose to do this because it is what I am good at, your son should chose something he enjoys (he will be doing it for thee years remember). In terms of what the RAF think, I wouldn't worry too much (I think they care more about your personal development, maturity/responsibility ect)
Maths, Engineering, Sciences, History, English, Foreign languagues, logistics/management are just some decent choices of the top of my head though. As long as he doesn't take some joke of a subject where all he does is drink for three years.