Then, I think the easiest solution here is to just stay home with your son, plan some activities with him and reassure him that your partner will call your son when he can, while he is away at his best friend's wedding.

To expect your partner's friends or your partner to accommodate your autistic son is unrealistic and not helpful to your son. Your partner can't control his friends' choices and it is not his wedding or your wedding. So, to blame your partner for his friend's behavior isn't fair to your partner. Just let your partner go to his friend's wedding, and plan activities with your son around your partner's absence, and let your partner call your son when he can. There isn't really anything you can do to make your partner or his friends accommodate your son's separation anxiety. You need to figure this one out yourself.

Your focus should be on how to accommodate your son, not on why your partner's friends won't tell him specifics for the wedding venue and reception. At this point, who cares about the wedding ceremony time and reception time.

Even if you knew these times, your son will still experience separation anxiety. So, that needs to be your focus. Not the wedding. But your son's response to being without his step-dad. How will you address it? Who is your back up, to help you, friend-wise, or family-wise, or neighbor-wise. Is there an emergency phone number you can call for help.

I know you are stressed about being left alone to deal with your autistic son's separation anxiety. If you really don't want to be all alone, then tell your partner of 7 years that you need him to either stay with you and not attend the wedding, or help you find people who can stay with you over the weekend who will help you with your autistic son.