HT, as an ex UAS Sqn Cdr x 2 I can understand your bewilderment. I expect things have changed somewhat in the last few years but the bottom line is still likely to be that there are a limited number of places on the UASs and the competition is extremely fierce to get in. Consequently each and every potential UAS member is scrutinised very closely and Freshers is only the first stage.
Without being there it is impossible to say how he "came across" but in my experience, if someone demonstrated a substantiated interest in the RAF/UAS at Freshers and was willing to at least try and make a commitment he would be considered a possible candidate and be placed in the "pool" for the next stage of medical and more formal interview. He would then be "scored" in order of merit along with all the other "potentials" and then it is down to where the cut-off line is, based on how many new students can be accepted in a particular year.
From what you say, your son would, IMO, have been in that second look pool unless he did not come over very well to those he spoke to. Believe it or not I have had students asking me if they had to have a sports car to join and had no clue about either the UAS or the RAF. That said, not much he can do now except try again next year, not much help I'm afraid but the moment has now passed.