Will this ruin an early Thanksgiving dinner?

Well my son is Autistic and he has therapy with the ABA which is Behavior ysis, on Tuesdays, from 6pm to 8pm. This is intensive therapy, with lots of reinforcements and motivating factors. This is what will teach my 2 1/2 year old how to communicate with PCES, or those picture cards to tell someone what he wants, since he is non-verbal at the moment. Anyways, my mom is planning an early Thanksgiving dinner for the family at my grandpa's nursing home for November 17, at 6pm. She wants me and my son to be there. My son has 3 therapy sessions on Tuesday, with the most important one at 6pm. I really don't want him to miss his therapy, because he just has until next August for this free, early childhood therapy, then he will get it in preschool, until he gets ready for kindergarten. Then if I want more in-home therapy for him, it's going to come out of my pocket. The insurance may pay for the speech therapy, but not the ABA. The regional office will give me some funding for it, but not that much. So it's really critical that he gets his scheduled therapies now while they're free. What should I do? I don't want my mom to think I'm being anti-social, but at the same time with my son being autistic and nobody in the family really understanding what that consists of, the Thanksgiving dinner could be a disaster, especially if it's nothing there that he likes to eat, and if he must be quiet and sit still for at least an hour. It could create a real sensory overload situation and cause him to stim, or start hand flapping and screaming and everybody looking like, "do you not know how to control you child?" When this isn't the case. But how do you explain Autism to a bunch of Adults in the midst of my son having a meltdown? What should I do?