He has no ownership stake. He isn't a director or member of the organization. The thing he claims is a contract he's party to, isn't.
I've skimmed the complaint now. There seems to be prima facie evidence of a contract there (though we'll see if the response suggests a lot of context was omitted). I find the Promissary Estoppel COA even more compelling, though. Breach of Fiduciary Duty seems like a stretch using "the public" as a beneficiary class. This isn't really my area, but I'll be mildly surprised if that one doesn't get tossed. Don't know enough about the Unfair Business Practices or CA Accounting requirements to have any opinion whatsoever on those. The Prayer for Relief is wild, but they often are.
So, once again, I have absolutely zero idea if OpanAI can be held accountable for not following their charter or not, but if they do, anyone can raise a complaint, and since Musk did give them money to save dolphins or whatever, he may actually be considered the victim.
There's a moral argument perhaps...but from a layman's perspective it's a really dumb case. Now, dumb cases sometimes win, so who knows.
I don't think there is such a thing. Once you co-found something, you are forever a co-founder of it. (Unless you have a time machine. Lacking such, nobody has ever un-founded anything, have they?)