Is sounds like it was truly a accident and no one was really at fault. It is good your son is Ok. Just a suggestion, I would go out and take a picture of the intersection from your son's view point and save it. Who knows what might happen and this would be good evidence in case the city cuts the trees and bushes in the future. They may also have some liability if the sign is obscured, which you might decide to pursue. It is much better to have evidence that you don't use than to not have it if you need it.