I have no idea if he has any standing or not, but from your reasoning it doesn't follow that he doesn't. If I put a box with a sign on a street "Donate to such and such charity to save dolphins", and you give me money only to later find out that I have nothing to do with that charity and your money will be spent on my new car, I scammed you, plain and simple, and you can sue me. Was this sign a contract with you? No. Do I become a stakeholder when I donate my money to charity? Obviously not. But it's a scam nevertheless. In fact, you don't even have to be a victim start litigation, but you can claim compensation if you were.
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.