Forcing/Pushing Socialization
Please forgive me if this isn't helpful -- I don't have an AS child, so I don't know. But it feels to me like the "make him socialize/get used to parties" thing is going very much against the grain for your son, and if it's that severe and stays that severe, it's hard for me to imagine that any coping skills he develops at age 6 will really come in handy. Adults who're that averse to social interactions don't generally wind up having to figure out what to wear to the corporate picnic, because they can't stand environments like that. Might it not make sense to allow him to develop socially at his own pace?
I can only say, that once I accepted my diagnosis and treats, and questioned the majority "But I need to go there, because otherwise someone could think bad of me." situations, and reduced them to the real necessary minimum - my life quality enhanced enormous. Before my life was s**t, now its great. What use do I have from tons of relatives praising my behavior ("Oh, your such a gooooood little doggie!"), if in the end that causes my life to be hell? Why should I care for the satisfaction of people, that in return dont waste a single thought on my satisfaction, but simply give a f**k about me?
My life has not become worse, by stop caring for such events, but has actually become enormous better.