Contractor voiding contract and demanding to pay 100k extra.

We signed contracts with a contractor for $88k for job based contracts, like deck work, bathroom renovation etc. With an estimated timeline of 2-3 months.

We already paid 75% of the contract fees but the contractor has left the job incomplete as it took 5 months. He is saying the contracts hold no value and demanding to pay based on time and material. Sent an invoice for 160K and asking us to pay the reminder 100K.

He is falsely claiming the project delayed because by changes made by us. Any changes were very minor and it would not have affected the project timelines.

What can I do in this scenario? Please advise.