Speaking as someone who was all-too-recently 14 and has known his fair share of sexually active 14-year-olds (NOT like that), if you don't let him, he's going to do it anyway. He'll even do it in military school, church camp, etc... If you call the girls parents, he will just find another girl to "do it" with... Though it might slow him down a bit. If you try to teach him to fear sex as opposed to teaching him how to be responsible about it, you will just do more harm than good. ESPECIALLY because it says a LOT that he trusted you enough to tell you the truth.
[My opinion: 14 is not too young, because he's doing it of his own accord... Though I would ask some general details to make sure he's not being pressured. People just make too big of a deal out of sex. It's a bodily function no different than peeing and pooping and fighting against it will result in nothing but pain and suffering. Fortunately, we live in a day and age where it can be done with little to no danger of STDs or pregnancy... THANK GOD FOR CONDOMS!]
EDIT: By the way... Did you mean Publisher or Unbaptized Publisher? If it's the former then .