Want to help someone with autism feel more comfortable in an unstructured social setting (like a get-together)? Consider giving him a role.
One of the difficult things about get-togethers is that there's often no structure to the gathering. There is unlikely to be a specific starting point (other than a general time), a theme or dominant topic, or a direction for each part of the evening.
Those on the autism spectrum often prefer topic and structure to loose social contact just for the purpose of spending time with others. The ASD individual would likely appreciate a defined role for the evening. Consider assigning him a task such as taking coats from the guests, keeping track of who gave what for a gift exchange (if a list for thank you cards would be helpful), or passing out beverages.
You could even assign him someone to "take care of." Often the ASD individual would like to be nice to others, but he likely feels uncertain about how to do that. If the host asks him to keep Grandpa company because this is his first Christmas without Grandma, that may be enough structure and direction for both to feel settled.
Keep in mind that one strategy for social success for the ASD individual may be to assign a role or task even when the gathering itself is unstructured.