Still picking on the Cories? Or is it more serious than that? I would just put him on his own as it sounds like he may be a betta that will not get along with others well. Did you say that he is red? They do tend to be more tempermental and more hard to get to put the others in with.
It may be that he is not going to have "company manners" no matter what you or I do with him. some bettas just do not get along with any others. They just do not live with them when they are young and cannot adjust.
With a red betta that has decided not to accept tankmates that is usually the only plan that works. They just do not know how to play nice if they do not want to. It really is not his fault, he is not able to do differently.
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