Hard to tell.
We miss lot of information.
Have you already delivered the product? Or, can you, as someone else suggested, disable it (for example if it relies on an important, hard to reverse engineer, back end, whose you haven’t given the source code)?
Is your client really a businessman? (Contrarily to what another person said, I would not call this “the way of businessmen”. Businessmen don’t do that. Not just because of decency – And they have more of that than people usually think. But because a reputation not to pay contractors is not good for them. Sure, in US, your, yet, current president is known to do so. But not anybody is president or even famous enough so that people are willing to take the risk not being paid because of the plus size of being contractor for a famous client. Plus, even Trump had to pay the price for his behaviour. He is also famously known to have some hard time finding contractors, even lawyers, because of his well documented habit not to pay contractors).
From which country is he? Even from another continent, western countries have laws mechanism to deal with that.
What kind of money are we talking about (if it is a few hundred bucks, you can’t probably hire a lawyer. But otherwise, there are lawyers in any countries where there is business)?
There are also debt collections companies everywhere (or even debt buyers in some countries. But that the last resort, since you would loose 90% of your money).