Ask a Business Lawyer. Get Business Law Questions Answered ASAP.
You could send him a letter stating that he breached the agreement by failing to provide the remainder of the consideration (i.e. the billboard, sign, etc.) and that you consider the agreement terminated. Along with this letter, provide a refund of the $600.
If he accepts the check as a refund you can rely on this as his acceptance of the termination of any agreement between you. Of course it would be better to have his acceptance of such termination in writing but it would likely hold up in court if later he attempts to claim that he owns 33% of your business.