This is best something you’d contact a lawyer for. It depends on your country and what’s in the current contract, but I suspect the following would be relevant:

• As you wrote the code, it should be yours (this is true for any written text, even if you don’t explicitly mention copyright stuff, as far as I know)

Once again, certain wordings in the contract may give you or the customer different rights, and local laws will apply. I definitely recommend you contact a lawyer for a definite answer. If you can’t or don’t want to pay a lawyer, I’d assume the code isn’t important enough. So you’ll have to weigh the advantages and disadvantages.

On the other hand, think about how important it is if he does use the code. You wrote it so you know the most about it. He’ll have no support, no further development (unless hiring someone else who has to work himself/herself in), etc. You on the other hand could use the code going forward.

This would be the issue of work for hire. Typically*, if you’re employed by someone and they have you make something, they own it. But you’re a contractor/freelancer, so that shouldn’t apply to you. Which is why most contracting work mentions something about this in the contract. If there was literally no contract and you just kinda legally eyeballed it, I believe you still own it.

I don’t say this often, but in this case, yeah, you should probably go talk to a lawyer if you really want to apply any legal pressure on anyone.

Also, it doesn’t matter at all if you’ve given him the source code in the past. Copyright doesn’t work that way. Even if we all had the source-code to Windows8, it wouldn’t mean anyone but Microsoft owned the rights to it.

*In most states of the USA. Offer not available in Antigua, who is specifically ignoring all US IP laws right now**.
**Update: In 2014 Antigua finally fined someone under the 2003 copyright act. Antigua and the USA are still in a tiff over online gambling, but the politics may be shifting.

This should have been specified in the contract (lessons learned…). In the best case for you, you would have specified that non-performance of contract terms by the client is grounds for cancellation, maybe a three-strikes rule or something. You would also have specified “intellectual property transfers on satisfaction of contract”; even in a cancellation, if both parties have lived up to those terms (meaning you’ve been paid what you are owed according to the contract thus far and have delivered accordingly), the contract is “satisfied”.

That then makes it clear that what you have developed now belongs to your client. That is, of course, assuming that other than the client being the source of insurmountable delays, the customer was in good standing with regards to payment for services rendered. If the client is withholding payment, you own the code until the matter is resolved to either mutual satisfaction or the court’s.

By the way, if the contract does not stipulate what portion of the payment you are entitled to and what you are not based on the work performed, then if you drafted this contract, be prepared to fork over every penny they gave you, and similarly, if they drafted it, you should be refunding nothing. As was correctly stated in a Big Bang Theory episode a few seasons back, ambiguity in a contract benefits the party that did not draft it, to the extent the law allows (the contract doesn’t have to specify the obvious, such as illegalities of law, but if you are neither required nor prohibited by law or contract to take a certain action, you may do as you choose regarding said action, so if the contract doesn’t stipulate settlement of partial completion, the money and the code will go to whomever didn’t write this sieve of a contract).