First thing is that you need to wake up to what has happened here. You cant hide behind "but he lacks capacity."
These vigilante groups do not contact men first. They just have a profile and wait. Your son contacted this random "child". These vigilante groups do not turn the conversation sexual. They wait. Your son turned the conversation sexual. It would be very unlucky that the first ever time he did this, it was a sting so he has most likely dont this before, with real children.
Doing that; finding a child's profile, starting a conversation, turning it sexual, arranging a meeting and then going out with a supervising adult shows he is very capable and able to be independent and understand what he was doing. He is a risk to all children, even if he only wants children after puberty, he is still a risk to children. And he made all those choices and took the actions necessary to get what he wanted.
He has capacity to manage all of that. You need to open your eyes.
He may not have the capacity to understand getting caught, getting in trouble etc. However, if that was true then surely he would have mentioned in conversation at some point that he was going to meet this boy? So he has hidden it from you. He knew it was wrong. He just didnt think he could get caught.
You've got to open your eyes. He needs to face punishment and the local community will of course be interested and want to follow the case and want updates on him. They will also keep their kids away from your home, so your younger kids will suffer.
Selling up and moving several towns over, without telling anyone from your home town, is the best you can do for the younger kids.
You need to remove all internet access for your oldest kid, take everything off him. And start looking into adult assisted living for him.