You should send him a notice for delayed construction work. When the builder completed only 25 to 30% of construction work it seems that the project is far behind the schedule mentioned in the brochure. This is an agreement and the builder is bound to perform his promise which has mentioned in the agreement. When the builder does not perform his part he cannot compel the purchase to make the full payment otherwise he will levy 15% interest thereupon. He has to complete the construction as per schedule to provide the flats within time. It seems from the current progress of the work that he would not deliver the flats on time.
You can claim the refund of the entire investment
In this situation, you can claim a refund under Section 18 of the RERA Act. In the said notice, you should mention that your construction work is not going as per the schedule so I could not pay the remaining amount as per the payment schedule given in the brochure. If he persists to make payment despite the delay in his project then you should file a case before the RERA authority under section 18 for the return of entire investment with interest.