The thing is it's a wedding. The celebrant has to be booked for a certain time, the reception venue will be booked for a certain time, food will be set for certain times.

It is entirely realistic to expect to be given ceremony and reception times, regardless of the overall situation. Otherwise how is anyone supposed to know what time they need to be at the venue?
This is literally information that EVERYONE is given when you invite them to a wedding.

Both partner and I are full time carers for my son (partner can't work yet due to previous workplace injury) so 3days is a LOT. I am never away for more than a few hours and partner rarely goes anywhere (not through lack of me trying), so this is really big for son and every professional we have involved has said to set times for phone calls and stick to them, to let son know when partner will be leaving and when he'll be home, and for partner to let us know immediately if anything changes. I know that son doesn't quite need that level of strict planning, but knowing what time the ceremony and reception are means we can plan ahead for even just 5mins to call between them (because the bride and groom will likely have photos without the bridal party at some point), or a short call before everything begins or during the reception. But we cannot even plan one single call because we don't know what time the bridal party will even be getting ready.

And again, what we're asking for is basic information which every wedding guest is given, not any special favours but basic wedding information so we don't interrupt their day.

Partner is getting a lift there and back but even that is an unknown. It's not unreasonable to expect someone to communicate with you about this stuff, especially when there's kids involved.
Partner is feeling incredibly left out and like he's just an afterthought in all this now. 😞