My son injured his shin one year ago today. They x-rayd it. Wasn't broken, said it was a contusion. Knot is still there. Not sore though. Y is this?

Variable. Some shin or other bony injuries can produce a buildup of bony material at the site that can last quite a while. It's similar to the knot that forms on a tree bark if it sustains a cut. It usually remodels over the years and disappears.