That's a typical story, has happened to lots of us. You have to see what you can get out of this. Not much point in spending a fortune unless you are going to be able to get it back.
My advice: Move on! If you can write good code, you will be able to find another project. If the code is yours as you claim, your best option is to licence it as GPL so anyone can use it and compete with him. Make sure that you keep all records relative to the matter, so you can prove that he breached his side of the contract by refusing to have your name in the credits. But don't distribute a hacked version of his program, because then it might be you that will have to compensate him.
My advice: Move on! If you can write good code, you will be able to find another project. If the code is yours as you claim, your best option is to licence it as GPL so anyone can use it and compete with him. Make sure that you keep all records relative to the matter, so you can prove that he breached his side of the contract by refusing to have your name in the credits. But don't distribute a hacked version of his program, because then it might be you that will have to compensate him.