Thank you for your query.Varicose veins
are usually seen the legs. What you are describing can be Mongolian spots which is usually present in infants and they disappear as an infant grows. If the spot is increasing in size then you should consult your doctor who shall examine it and give your the right guidance.
I hope I have answered your query to your satisfaction.
Wishing your son all the best.
Rajiv K Khandelwal