Dispute with the contractor and abandonment of the project

I am a property owner where my contractor walked away from the job introducing an un-approved change order. He has already collected 85% of total payment from 7out of 8 milestones but only completed 75-85% of each milestone work. At the final stage he introduced a change order for the work done in 4/5th milestone and 2.5 months before. That change order was never known to us and never approved so its payment got denied by us. When we told him to complete the paid milestone job immediately he left the work (He threatened many times before to quit the project too ---red flag but we were stuck with a contract). We paid 7 milestones 100% payment in good faith but we got cheated in return and now we don't know how to proceed. If we sue him, we will end up spending more than he owes us. Small claim court is too small an amount to cover. Our project has stopped for the last one month and we don't know what is the best way.