Nina, I am SO sorry your son was let down so badly. He may have special needs, but any of us, expecting others in support, would be so disappointed, and feel abandoned if people ditched an idea at the last minute without ensuring our group knew it was not going to happen.
I'll just add a big 'well done' to your son for entering into the spirit of the event, I do hope he has got over feeling abandoned, because that was what the group did. They allowed one vulnerable person to feel alone.
Shame on the group for pulling out without notifying your son. Your son made the effort nina, For that he should be very proud. Good on him!
The group has questions to answer.