You state he
Complains about his knees and then remark he has a bulging vein behind his knee.
I would tend to think the vein is not the issue as you reveal if I understood he only has the vein on one limb.
I would make sure he ices the knees. I would examine his feet to check if he prorates excessively or if the foot moves towards a flattened position as his puts xtra pressure on knees. Growing pains as well.