Hi
Welcome to the forum!
Well, if your son is very fair skinned and the skin is thin with less of underlying fat, then veins can be visible and look prominent whenever they are engorged with deoxygenated blood (has a bluish tinge). This is a normal finding and nothing to be perturbed about. It may look unsightly and cause stress therefore. This can be obliterated by building up the muscle mass and increasing the skin tone. A slightly tanned skin too may help. Maybe your son can join a trained physical instructor.
However, before he does that, peripheral vascular disease, faulty deep veins’ valves and high blood pressure as the cause would need to be ruled out. Kidney function tests should also be done.
However on net and without examining it is difficult to give a confirmed opinion. Please consult your doctor about these possibilities. Hope this helps. Take care!