Verbal agreement about a car

[DR2_17]

Virginia. Hi, i'm new on here but i have a question about a car: In march my mother called me telling me my older brother needed help with his bmw, he couldn't afford the payments and go to school. Me and my brother made an agreement that i would take over the payments (insurance and all) and he would get an car to get back and forth to school. He assured he would'nt take the car back. I have made the payments on this car on time with out any problems and well i guess he feels he can afford it(or just misses the car) and now wants it back. The car is still in his name but i have been making payments on it and we had a verbal agreement that he wouldn't take the car back. My mother, father and a friend have all witnessed him telling me that. What can i do?