we find people doing subjects such as History or Geography can offer more commitment to the UAS than Engineering
Well so much for a fair selection process that bases its decision on talent rather than fitting a pre-conceived idea. However, it is a fair reflection of an attitude that appears to be prevalent in some areas of the RAF that expects the individual to commit 110% but with substantially less commitment in return. But it's not new and we had a number of engineering, physics and hard sums students on my UAS in the 90s who had a harder ride than those of us doing the likes of Geography and History. Apart from a couple of female engineering students for whom sporadic attendance was overlooked and they were actively helped to stay on the Sqn and fit it in around their studies.
Given the importance in the longer term of getting a decent degree vice running around muddy fields doing 'leadership', I'd say your son had a lucky escape, especially as he will now more than likely be paying for his degree for years to come. University is a place for learning and self discovery, of which the UAS can play a part, but it shouldn't be everything, and whomever interviewed your son needs to wake up to that fact.