The only recourse at this time is for a complaint to be made by the Clerk, Councillors or a member of the public to your Monitoring Officer. But that will not get you very far as the majority of MOs are toothless in these matters.
I believe that the membership to external bodies is agreed at your annual parish council meeting, which is normally held in May. So you might have to wait to remove him from his representations. The only possibility is that you haven't had this meeting yet due to Covid and so you could hold it next month...? Legislation has allowed for the May 2019 meeting decisions to be carried over the May 2021 but it is unclear if allowance is made to have this meeting at another time in between these dates.
Sorry to be the bearer of bad news...