Unfortunately, you are unlikely to get this contractor to finish the job, or provide you any of your money back. I would treat it as a lesson learned, one that most investors go through early in their real estate career.
Construction attracts many deadbeats, since the barrier of entry to many trades can be so low (ie painting). There are also many amazing contractors, who are wonderful to work with. It can just take some time and mistakes to find the great ones. I lost a few thousand dollars at first on contractors, before I found my current team who are fantastic.
I always pay for all materials. I provide home depot gift cards, within $100 over estimated costs. That way no contractor can complain they are out of pocket. I then pay only after work is completed. No half up front, or on last project the painters had completed 2 rooms out of 8, and claimed they were half finished.
When contractors start complaining that they need money up front, I tell them they can take the job, or leave it. They provide a name and a license, I provide a building. I have far more to lose by being shady than they do. On a larger job, we will break out cost by task, and pay when each task is complete. This can get you a better price too, as the contractor is then not building in contingency for unknowns.