Cracks occur in slab concrete for a lot of reasons. In your case, the new house is protected against defects by a builders license. He has no interest in finding faults that will cost him money, and he may be right that the cracks are inconsequential. Virtual construction consultants out here in the matrix are in no position to judge the situation.
I recommend you hire a competent and impartial building inspector that holds a contractors license. Insist on a written report. He will give you a qualified opinion that can be used if you need to address the matter as a lawsuit. This will also start a paper trail that establishes the initial discovery of a defect, that may or may not need repair at this time; but may give you better options to pursue the matter (again legally) later on should the situation become worse.