The old saying of course is "get it in writing". Like unregistered copyright, verbal contracts are technically valid too, but both are extremely difficult to enforce without evidence. I know nothing about law, but my guess is that without hard evidence, your best chance might be a discussion and negotiation, perhaps extra payment, to settle it with the client, out of court, without pissing each other off. That would likely be about who needs the other more, him or you?
There is info on the web, dunno how good it is...
photographer's contracts:
9 Free Photography Contracts from Docracy
small claims:
http://www.courts.ca.gov/partners/documents/smallclaimschecklist-en.pdf
I am not pretending to know anything about law, but is it about copyright or about a verbal contract? A lawyer could advise, perhaps about any possible witnesses or evidence, and about your chances. A lawyer can write a letter threatening court action. The client could see it as the bluff it is, but still, if it continues, both lawyers will get paid either way, so the defendant may consider complying to be the lesser cost. Seems like it is either worth this cost to you, or it's not.
You could have put many images for a big job(s) on a DVD, and registered copyright of all on one form for one $35 fee. That's very little cost for a big job. Then your evidence is the registration number of these images on the DVD in possession of the Library of Congress with the date of your claim of ownership. Then there is not much dispute possible, the earliest date wins. However, this is a big court and a pretty serious affair, they say legal fees are quite staggering to defend that, but you can get costs and punitive damages if you win, and you should. And possibly might even collect it.