is the autism thing a red herring?
what is the curriculum for the trip? if it is a 'future leaders' thing are they going to be expected to be giving talks and presentations, carrying out activities/ taking full part in anything that your son might struggle with?
i don't really understand the scheme (is it a one-off or part of an established programme), but presumably there has already been some sort of entry criteria that your son has passed? so he has already proven himself to be capable of becoming a 'future leader' in the terms expected of the organisers? (or was it just a 'sign up' thing, rathert han a selection-based thing?)
do you think they are concerned that his more retiring personality might mean he finds the tasks/ activities very difficult? is it more of a leadership training/ ie confidence building trip? so something he will in fact, benefit more from than some of the other participants who fit the 'future leader' role in a more clearly defined fashion according to the organisers?
without knowing more about the trip, i think it's difficult to say what the organisers reservations are, tbh. is it possible that they are using 'autistic tendencies' as a way of (clumsily) expressing that they feel he might struggle? will his lack of confidence cause a problem for him being away from home with a group of 'go-getters'?
you catch more flies with honey, and all that. i wouldn't start throwing disability discrimination around (particularly without a dx lol) until you have to. start nice and ask what their issue is, exactly.
if it is a selection based trip and for some reason your son does not fit the selection criteria (whatever it might be) then it might have nothing to do with autism (although why he's have got to that point in the process i'm struggling to work out).
it might just be that the organisers need more information, particularly if he is likely to need more support on the trip - in which case, the more you can reassure them, the better.
if he struggles with his confidence, this would be an amazing opportunity, if he's keen. hope it goes well x