It is good to let your son know what to expect, which is probably lots of over-excited children running around and making a lot of noise. You may be able to find out more by asking the host.
At this age it should be OK for you to stay at the party with your son, and that will probably be your number 1 defense for making sure he can handle the situation.
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Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).