I’m holding 10% due to bad work which he said months ago he would correct before he sent his final bill and I’m positive he has no intention to do at this time.
If an interest clause is not in the contract or any attached terms and conditions then generally no, you cannot be required to pay interest unless a court orders you to do so. If the contractor takes you to court he can secure an award of post-judgment interest at the then-prevailing legal rate.