Literally, anyone can sue anyone for anything in civil court. But: in order to win, the plaintiff must convince the jury (or judge) that they have suffered damages. The loss of "potential profits" is barely rational. In personal injury or wrongful death cases (among others), future potential earnings can be awarded. But again, he can't really show a loss or damages for what you won't be doing for him based on what you have done for him, and mostly for free, it sounds like.

It's good that you have a paper trail and evidence that you may need. Keep track of everything. You do have a contract: it sounds like it is mostly verbal, with details in emails. It's a binding contract, but if you have finished 99% of the work, few would say you are breaching the contract. There is no implied contract for future work based on the contact you have now.

If he files a lawsuit as pro se (representing himself), and the lawsuit isn't immediately tossed out, you will need to get a lawyer to fight it. If that happens, Google for free/low cost legal aid organizations in your area. Many NGOs offer help with civil (non-criminal) cases. Or find a lawyer who gives free first consultations; you may find one would who love to do a "smack down" on the guy.

If he can't file as pro se, he must find a lawyer who believes they can win. He may be able to find a lawyer as psychotic as he, but I doubt it; there would be too much at stake for the lawyer's reputation and possible disciplinary action by the court.

If he files in small claims court, you have the advantage because small claims are informal, and you will be able to argue your case without a lawyer, and he will likely make a fool of himself. But, he may not even be able to file in small claims if he can't show real damages, i.e. a monetary sum to enter on the filing. If he doesn't correctly file in small claims, his lawsuit is tossed out.

You could try and head off any legal actions on his part by writing a firm letter explaining that he doesn't have a case, and  mentioning that you have legal representation, but get help doing that, re: the legal aid methods mentioned above.