It's such a little change medically, but with so much difficulty for a kid stretching into adolescence, and not doing too good to his confidence either.
As the proction of the male hormone revvs up , some of the precrsors are diverted into making the female hormone that causes this embarassing change that shall soon vanish with the male hormone driven pectoral bulges will be drowning all the effects of a miniscule amount of estrogen.
Next Steps
Do nothing. 3-4 years is the max duration of awkwardness disabling him desires of Salman style shirtless show! give him confidence!
Health Tips
do not touch or manipulate the mass, as it can get infected and as some times painful.