legal responsibility

My 17 year old son who had been living with his mom had moved in with me. His uncle on his moms side called while I was at work and told him he would get him stoned if he came in town and worked on his computer.
My son took an uninsured truck into town and when he couldn't figure out what was wrong with the computer his uncle slammed him around and told him to get the f--- out of there. Scared and stoned he tried to get away and backed the truck into his uncles p.o.s. car
I realize the insurance was my responsibility and it wasn't insured because it never went anywhere other than directly across the road into the woods for firewood.
Basically I am wondering why if its the bars fault if someone leaves the bar drunk and wrecks why isn't it his uncles fault for getting him stoned?