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Hello and thank you for your question.
From what you have described, they are most likely "stretch marks" or striae. These are very common and tend to develop during the growth spurt years due to rapid skin stretching. At your son's age, he is most likely experiencing pubertal growth that can continue through age 18 to 19 years old.
Unfortunately, once striae are present, there is not much to do to get ride of them. There are some dermatologists that do laser treatments in adults, but I have never seen this done in a child. The best way to prevent your son from developing more of these marks is to moisturize his skin daily. He should use a good quality moisturizer while his skin is still wet after showering on a daily basis. This will increase his skin's elasticity and prevent further striae.
I see that you are still offline. If you have further questions when your are back, feel free to ask.
I understand. Most of my male patients don't like using lotion, but I encourage them to try it anyway.
All the best to you and your son!