My 14 Y.O son is not obese but really skinny. His veins bulge out a lot when he's leaning on somthing with his arms or legs. Why is this?

Normal. blood becomes backed up in his veins because of the pressure and slowing of venous return to the heart, gets relieved with correct posture, by being skinny it becomes more apparent, but everybody has it, skinny is better than being obese or overweight, but being over-skinny isn't healthy too, unless that's his own built, so long he is healthy don't worry, take care.