What Causes Blue/purple Stretch Marks On Lower Back?

My 17 year old son has these blue/purple looking stretch marks on his left hand side lower back. He had around 8 last week but has double amount now. He is 5ft 9ins and very skinny. He has no pain but they dont feel like stretch marks to touch. there is no redness or silvery strtching around the skin area. I am a conserened mother as he also has a poor diet. Thank you Lee-anne