J.T.
Trying reminding your son that there is NO such thing as a looser when someone is practing to become their best!
I tell my children a looser is "someone who never tries!"
Teaching you son to respond in positive manner when someone is negitive will take him far in life.
Sometime giving him the rights words to respond will help them to learn how to find the right words later.
Like helping him to respond to the boys "I am learning how to play; today I may not be good , but one day I'll be Great!"