Individual Contractor Mr Perumal - Poor Quality of Construction and Significant Damages within a month

During the first week of September ****, we had issued a verbal “Construction Labour Contract” to Mr. Perumal (Mob: +** ***** *****, +** ***** *****) residing in Chunchghatta area, to build an extended accommodation for personal purposes on the *st floor at the above- mentioned address. Major construction works were completed during the last week of November **** as we were approaching the House-Warming Ceremony on November **, ****. There were pending works though, which the contractor agreed to complete it later, after the ceremony.
While we were just yet to complete a month after the House-Warming Ceremony, we observed a significant crack in the main wall and noticed poor quality of construction work in other places too on the newly constructed *st floor premises. I just noticed this and informed the contractor on February *, ****, in view to seek his suggestion and advice to correct / repair, for which he mentioned to come in person and look immediately, but he only came on April **, ****, after ** days from the day I notified him of the damages caused to the building.
But, while he came on Wednesday, April **, ****, and witnessed the damages caused and other allied issues on the *st floor, he started to defend saying everything is fine and there were no issues at all. Further, raised his voice and yelled at me using abusive/vulgar language outside the house premises which was witnessed in the neighbourhood.
I had called *-* other contractors and showed them and they mentioned that this is not just a seasonal air crack but a clear separation crack between the brick wall and the cement block wall. The contractor should've used iron rods, mesh to join the same and build the wall, but it is sheer negligence and careless which has led to this permanent damage.
With the rainy season approaching and if the water sinks into the wall from this crack this would cause the whole building to lose its strength and cause greater damages to both *st floor and the ground floor.