landlord tenant Q Must I return this money?

Tenant (room for rent) applied for rental of room in my home. He held out that he is a physician. He put down first months rent, and agreed to last months rent on the 1st of the month upon moving in. After I took down my rental sign, per his request in writing, and after I stopped advertising, 2 weeks later he decided he didn’t want to move in. He was offended that I researched and found he was not really a doctor so he wants out of a signed contract that stated he agreed to to pay 1st (he did) and last upon moving in. Now since I told him I know he is not a doctor, he wants his money back and is threatening to move in and only live the month he “paid” for and not 1st last and a deposit as he promised. He wrote to me today and said he plans to move into my house on the 1st and pay none of the other promised money. What can I do? I lost the rent for that month since I devoted the room to him, and now he is trying to change the terms. What should be my reply? Scared and upset.