I agree with divine1966 that it's unrealistic for you to expect your partner's friends to accommodate your autistic son's needs regarding the timed phone calls. My nephew is autistic and over the years whenever my sister or BIL were invited to a wedding, one of them always stayed behind with my nephew and neither of them resented the other one for going to the wedding without them.

You don't need to break up with your partner over this. I'd say, you have choices:

1. Make it a fun family weekend trip so your partner can go to the wedding. This supports your partner since it's his best friend, and you and your son can have fun doing something out of town. I don't know how severe your son's autism is.
2. You can stay home with your son, and ask someone to stay over the weekend to help you manage your son's separation anxiety. Make arrangements with your son's therapist or doctor to be on-call for you, should you need advice.

Having a child with autism isn't easy. I know this, as an aunt to my nephew. I've watched my sister and BIL juggle social commitments over the years sometimes struggling with resenting each other, but always coming back together in love and support b/c of their shared love of my nephew. This weekend my BIL and his other son are on a fishing weekend, while my sister stays home with her autistic son who has the flu.

I don't think you are the bad guy, Foreign_Soul, as long as you look for the compromise in every situation where everyone's needs get met somehow. In the case of your partner's friends' unwillingness to be straightforward with him about their wedding ceremony and reception time and venue information ahead of time, which is normally printed on the wedding invitations, I'd say discuss alternatives with your partner. You and your son could go with him and make it a fun family weekend trip, so you can still be together. Or, let your partner go, assure your son that his step-dad (your partner) will call him when he can, while he is away at the wedding, and set up activities with your autistic son to do, to address his separation anxiety.

I think the real issue here is your autistic son's separation anxiety. The wedding presents a challenge b/c it takes away your partner from your son, so you are naturally worried how your son will handle being away from his step-dad. How have you handled their separation from each other in the past? Have they been apart before? What sort of things worked? Maybe use those sort of things again, for the wedding. Don't let this wedding break apart your family or cause any more distress. Try to find a way to work with the circumstances, so that both you, your son, and your partner will all be happy with the outcome.