Welcome and thanks for posting your query to Healthcare Magic.
As per your query your son has small spot on the bicep that looks between blister
and is not painful.
You have not mentioned the exact time period since the spot is present.
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'.
These are generally formed due to injury by friction or any time during playing in this age group. The blood gets collected under the skin and we medially call it as "hematoma".
Ask your son for any injury over that site. You can wait for next few days and the swelling will be gradually subside.
If the swelling persist or become larger then it is better to consult a dermatologist
for best guidance.
Do not scratch or even try to touch the area unnecessarily. Due to touching or scratching or pinching the area, there can be secondary infection to the area and can aggravate the problem.
Hope these information will help.
"With good health wishes"
Dr Sanjay Kumar Kanodia
MD (Dermatology & STDs)