Hi.. I Have A 15-year-old Son, Who Suddenly Appeared Light

Hi.. I have a 15-year-old son, who suddenly appeared light brown bruises 2-3 of them, along his spine. Sometimes they get disappeared and some others appear either on the same place or a little more up or down. Other than that, he doesn't have any other symptoms (pain or fever, etc) and these bruises seem to come and go for the last 3-4 months. Please, I would like to hear your opinion on that. Thank you in advance.
Afrodite