Welcome and thanks for posting your query to Healthcare Magic.
As per your query your son have greenish spot. IN your query you have not mentioned the exact area where the spot is present and the time period since the spot is their. As per your query the greenish spot also have reddish color along with of the spot.
Most commonly these kind of spots are formed after simple collection of blood after any injury
or due to friction which we call as "bruises'.
As these bruises changes the color it is first red, then blue, black and lastly green.
Ask your son for any injury over that site. You can wait for around 7days. If the color is changing then is nothing to be worried but if this persist then it is better to consult a dermatologist
for best guidance.
Hope these information will help.
"With good health wishes"
Dr Sanjay Kumar Kanodia
MD (Dermatology & STDs)