I ditto the last post, please find out more detail from the school. It might be that you can help negotiate a way for him to attend. It will be hard not to get defensive, but if you can manage that it may help ease the situation.
If not, I also think that if you or another family member can go on the trip, it would be a benefit for you. I really couldn’t go to as many classroom things as I wanted to, but it was such a huge help for me to see my kid’s classmates, see my kid interact with others, see the teachers while they are working, etc.
It’s hugely important to find out what he’ll be doing if left behind, and who else might be left behind too. Maybe you could give him a reward for good behavior away from the class, so that he’s able to feel solid about the day after. Good luck!