I'm not expecting them to accommodate my son's need for timed calls, I'm expecting them to provide my partner with the same information which they've given everyone else so that WE can work around THEM. If I expected them to accommodate his needs I'd have already organised times, regardless of when their wedding is.

What we are trying to do is exactly what we've done other times, it is what works, and it's what all professionals involved have said is the best course of action.
Not having details (which, again, everyone else BUT my partner have) is going to cause serious issues. My son needs to know roughly what's happening, which isn't putting anyone out at all (again, same information every other invitee has). Not being able to give him this information causes him such severe anxiety that he has meltdowns, which puts both him and myself at serious risk, especially as son weighs only a few kgs less than me.

It literally does not put these people out one freaking bit to tell partner what time their ceremony and reception are so we can make at least one time that doesn't interfere with that. I'm trying to work around them but without basic information I can't.

Seriously, who plans their wedding and doesn't tell their best man what time it is?!