Call them every day and politely but firmly ask how it is going. In California, in addition to the check crime, theft of property or services above $950 is "Grand Theft".
Also post his name and a scan of the check and the contract on the internet. Be sure everything you state is factual. Also, since you have not been paid, you have a civil claim against him for copyright violation. File a take down notice with his site's host.
If you want your money back, you will have to go out to California, get a claim against him, then locate and seize his property for the debt if you can. That's going to cost you more than $10,000 to do of course, so whether you get justice is up to you. Just don't expect you'll ever see that $10,000, the value of that work is gone.
It's possible you have a signed contract stating that the venue for disputes is your state of New York. If so, that is easy to get a ruling against him since it's unlikely he'll show up for the hearing and you'll get a default judgement. Then you take that judgement to California and try to locate assets to seize. Since you didn't mention this, it is likely you have no such contract and you'll probably need to fight the whole thing out on his turf. In the future be sure to specify that all legal conflicts must happen in your area.
Also very important. In the future you do not do one day of work until you have 1/3 cash up front. The contract then specifies you get the second 1/3 at a key milestone, and then the final 1/3 when you actually transfer the code. If you had done this (which is very standard with projects for unknown clients) then you'd only be out $3333 right now.