If you had no knowledge of the odometer roll back, then you should be successful in any lawsuit he brings. There is no "strict liability" for odometer discrepancies as used cars are typically sold "AS IS." He should have run the report before he bought the car, not after. On the other hand, if he is able to show that the dealer that sold you the car sold it to you with a higher odometer reading, you could be in trouble. That, of course, would mean that you fraudulently changed it.
I think it is especially suspect that he wants to drive the car for 6 months while he has asked you to repay him. It sounds very possible that the Carfax report could have been faked and it is he that is scamming you, not the other way around. If he has filed a small claims action, let the judge sort it out. At the hearing, be sure to bring all your paperwork associated with the vehicle, including everything you received from the dealer who sold it to you.
The answer given does not imply that an attorney-client relationship has been established. Your best course of action is to consult with an attorney by phone or personal meeting. Contact your local bar association for a referral.