A lot of this depends on him, how he deals with things and how well he talks about things, how previous contact has gone, and her - how predictable she is, etc. So lots of unknowns. You have been thrown in at the deep end. It is very difficult to give advice on this in a vacuum as how he is and how he reacts and so on will affect how you play it so these are just ideas
Is there a chance she won't make the date? If so, to prepare him you could say that you think it is going ahead, you are not certain, that the weather might cause a problem or other reasons why it might not happen, say that if it doesn't go ahead you will re arrange etc - so that he is prepared for it maybe not going ahead. Depending on how he reacts, he might need more talking about things sometimes being unpredictable, some people being more predictable than others, etc.
Hopefully he has been talking about it to you ahead of time - the more he talks about how he feels the better for him and the better you know how he is feeling, which helps you help him.
I am assuming that if it goes ahead, you will be present the whole time, so you will start getting to know her a bit, which will be good and useful.
Afterwards let him feel and act out, and don't take anything personally, and try to connect and absorb some of how he is feeling.
If he appears unaffected, completely fine, not unhappy, as though it is no big deal, it might well be that as soon as there is even the smallest amount of stress - knocking something, a problem at school, struggling to do something - it all comes out at that point. I think it is important to comminicate to him that what he is going through is hard, not his fault, and that you will do your best to make it as ok for him as possible.
It might be that going out for the day somewhere afterwards, if you have a beach or forest or something a covid-permitted drive away, not too many people, a bite to eat - a change of scene and walk in fresh air can help sometimes, if he would feel safe doing that with you. It might help him open up about things on his mind.
I hope it goes well.