They could be stretch marks...My son has horizontal lines on his back...some purplish, and they fade...just like stretch marks from my pregnancies....he is 15 and 6'2" so he's had some pretty big growth spurts....our Dr. said they'll probably fade away considerably over time:)
Hi!
In all probability your son has either eczema or dermatitis or psoriasis. Very dry skin due to Vitamin A deficiency, hypothyroidism, Sjogren's syndrome, and diabetes is another possibility. It is very difficult to diagnose this on net. Do discuss this with your doctor and get your son examined. Take care!