I'm not a lawyer, but I've seen one on TV. :D
When you write a program for another individual or company, then unless some other agreement is made, that individual or company actually owns the rights. You essentially sell your rights for a payroll check or a fee.
The fee in your case is credits in his program. He failed to uphold his end of the agreement and therefore the rights to the code you wrote revert back to you.
But that's only for the parts you've written. You still have no rights to anything anybody else wrote, nor to any modifications he might've made to your code. So if you are distributing the program without his permission, you are breaking copyright law.
The best you can do is get a court order to force him to remit payment, i.e. put your name back in the credits, or to quit using the code you provided him.
Ultimately, it comes down to just what the code is, and how much it is worth for either of you to pursue legal action.
However, even though he might not ever pursue legal action against you, it still probably wouldn't be advisable to redistribute his program without permission. If down the road, there is an opportunity to do some real work for real money, you might lose that opportunity if the person fears you might take his program simply because you coded part of it.
When you write a program for another individual or company, then unless some other agreement is made, that individual or company actually owns the rights. You essentially sell your rights for a payroll check or a fee.
The fee in your case is credits in his program. He failed to uphold his end of the agreement and therefore the rights to the code you wrote revert back to you.
But that's only for the parts you've written. You still have no rights to anything anybody else wrote, nor to any modifications he might've made to your code. So if you are distributing the program without his permission, you are breaking copyright law.
The best you can do is get a court order to force him to remit payment, i.e. put your name back in the credits, or to quit using the code you provided him.
Ultimately, it comes down to just what the code is, and how much it is worth for either of you to pursue legal action.
However, even though he might not ever pursue legal action against you, it still probably wouldn't be advisable to redistribute his program without permission. If down the road, there is an opportunity to do some real work for real money, you might lose that opportunity if the person fears you might take his program simply because you coded part of it.