By the sounds of it, this is a simply case of breach of contract. The builder agreed to pay you for your services, you performed those services, and now he doesn't want to pay you so he's making excuses. From what you've told me, you should be entitled to compensation / payment for your services. There are two main options you have:
1) Have a lawyer send him a Demand Letter i.e. a letter demanding he pay you lest you commence legal action against him. The is generally a good first step because, if the builder complies, then it is a cheap and quick way to get your money. Even if the builder has previously ignored your demands for money, it carries more weight when the threat is sent by a lawyer on the firm's letterhead and suggests legal action if the demand is not complied with.
As there is no guarantee the letter will work, you may have to resort to the following / next option (or you can skip the first option entirely if you don't think it will work)-
2) Sue the builder. Assuming the amount is for less than $25,000, you can sue him in Small Claims Court, which is a relatively simple and cost-effective venue for litigating a matter. If the amount exceeds $25,000, you can still sue, but it would have to be in Superior Court i.e. the higher level of court, which is more comlex.
Hopefully that answers your question.