He can sue, but who and for what? Essentially the community is showing him a finger even if he didn't break the law. He will keep the money, but they simply won't be doing any business with him.
What makes it weird is that they can't just buy back the shares, because that wasn't in the contract. They have to convince the body controlling the currency, which backs the issued shares, to essentially invalidate and reprint a pallet of virtual cash.
Although the DAO shareholders don't necessarily have to participate in the fork for it to succeed, publicly advocating a fork, before the fork, in order to nullify a contract seems like it could be considered a breach of contract.
Next thing the IRS will go after him for the corresponding taxes on millions of dollars of crypto currency