Hi potter638,
clubs eh? From your story I understand you've been involved with Scouting yourself for several years. From that period, do you know any of the people involved in the Scouts group your son is attending? I would ask them what his poor behaviour was. Were you made aware after the first time or did you only hear after the second time when he was excluded? If so, why? Since he isn't the only scout excluded, was the poor behaviour a group thing? What made him then stand out that you have to attend whereas other parents do not? Did he lead the group? Was he the one caught in the act? You don't have to post the answers here, it's none of our business, but you need to know every detail. Letting them explain what happened, and why they decided on exclusion, verbally in detail to you also gives them time to reflect upon the whole business and it shows your interested in your child.
My middle son also had trouble in clubs (sports, not Scouts) and I'm always there too. I wasn't asked, but between the lines it was clear they liked me to be there (this is about 4 years ago now). It's going better now and perhaps I needn't be there, I'm usually just pottering about with the little one, so I'm there, but not really involved. My son also has extra help in school, so for us it's not just in clubs.
It's tricky, because you don't want them excluded (at all, but definitely not permanently) and you do have to support him in achieving that. I've also been a scout leader (a lifetime ago) and that just grew from being a member to being a leader and we in our late teens/early twenties would not have had a clue about adoptive children. We did have one (internationally) adopted child in the Explorers though, but she did not have any behavioural issues that I recall. I'm saying this because although we want teachers and club leaders to be empathic towards our children, it's not so easy. Until I adopted and lived this, I didn't have a clue either and even now, it is not easy day in day out to deal with (some of) our children, as I'm sure you know too.
I would speak to the Scout leaders to get to the bottom of what happened, who did what and take it from there. Obviously something happened that you'll need to speak to your son about and only you will know how and when to do that. As I'm sure you will, also fight his corner with the Scout leaders, but don't make it adversarial. At the end of the day, (I think) you want him to be in Scouts because (I think) he wants to be in Scouts. Your son also needs to understand that if he wants to be free of you, he needs to make sure he doesn't get into big trouble. It may also be that he deep down does not want to be free of you and would like you there.
Good luck. I know it's something you don't need and it's not easy.