Bad Service, Delays and Breach of Agreement

My Husband and I agreed with Supreme Builders, (Ian Lodweyk) for completion of Renovations at our house, in Parklands. This was supposed to be a three week project. As I write, the project is not finished and we are now in the 8th week. As agreed, we paid the initial deposit, the work started. A second payment was demanded soon thereafter, on the basis that more supplies are needed. We paid. Not long after that a third payment was demanded, at this stage, I was concerned that the demands for money were too quick compared with the project. My husband addressed this with Ian. Ian assured him that all is well, and that he needed to get supplies. As we wanted to see this project finished, we obliged. After this payment progress slowed down significantly. We then had many talks with Ian about our concerns. Instead, he demanded the rest of the money, we refused. As I write this, we have paid him 75% of the quotation, and the work that has been done (some of which is of poor quality) is around 50/ 60%. The huge concern we have is that, the supplies that are outstanding for the remaining work are more costly than what has been used and done so far. We stand to lose a lot. Please Help.