if I may ask, what are you hoping to achieve by trying to regain contact with someone whose fundamental values differ so greatly from your own? They may be your son, and you can still love them from afar and the person they used to be, but I think you shouldn't compromise on your principles and break bread with him unless he himself wants to be someone worth associating with.
It's probably much easier for me to say than it is for you to do, but I would strongly suggest actually not approaching them at this get-together, and instead seeing if they approach you first. If they don't and just pretend you aren't there, I wouldn't say they're someone you should be overly concerned about connecting with.