All you need to do is assemble a quotation for the additional work and send it to him. Estimate the number of hours it'll take you (+ 25% since these things ALWAYS take longer than anyone thinks) then send it to him for approval. Keep it polite. Tell him if he wants the work done, all he has to do is approve the quote via email and you'll jump on it, with delivery by date x.
It'll accomplish a couple of things: (1) indicate that the party is over, no more "minor mods" for free, (2) shows in a professional manner that you disagree with his definition of "minor" without making a big deal of it, and (3) serves as a defacto contract that you can wave around later for payment. If he agrees, and you deliver, you're owed the money. Period.
Of course this won't replace a real contact if/when that comes, but it'll cover this small bit of extra work.