How do you teach a kid to be confident not cocky?

My son, who is 12 & very athletic, is also pretty cocky. He brags & rubs in his successes & unknowingly (i think) makes others feel less than adequate. I want him to feel confident, because he really is good at sports - he has the natural ability & the give 110% attitude to be amazing athlete well into his high school years & hopefully college - however.. i don't want him to be viewed as cocky. I am sure it is mine & his fathers fault for always telling him how wonderful he is his entire life, he is the baby of 4 kids & we probably spoiled him & doted on him...
so.. how can i now teach him modesty w/ confidence?