Suggest Remedy For Bruises In Lower Back

My 4 year old has kept nickel size bruises down his lower back for the past year. I mentioned it at his 3 yr old check up and he is very active boy so dr said probably completely normal but he always has them. Sometimes there seem to be a only a few around his spine and the Sometimes more of them all around the lower spine but I never see them purple just always a light brown color like a bruise in final stage before it goes away. He is a very rough boy. Falls on ground on purpose, bumps into everything, climbs, jumps etc. so I just keep thinking he is doing something to bruise his back but he never complains. This has been consistent for a while and I am starting to worry.