I'm 17 and got in car accident.?

I was recently involved in a car accident. I am 17 years old and i wasnt on anybodys insurance, and i borrowed my brothers car to go get my girlfriend. ON my way back some guy sped up came into the near lane and incidently i hit the side of his car. I know for a fact it wasn't my fault, and he accusably made up lies such as, he beeped his horn, he has a witness, and that i didnt see him. I checked both ways twice and i know for a fact he wasn't there, so he had to be speeding and he switched lanes. Since he is a lawyer he thinks he can easily boss me around and sue my brother for 4500$ for car damages when his door has a meer dent on the side not serious nough, and if we dont pay within 2 days he wants to sue for 25,000$. What do i do?? =S