(It only happened once, but I'd like to know some techniques for keeping it from happening again.)
He is, for your purposes, a babe in the woods. Coddle all you want. I wouldn't suggest actually condescending to him, if only because condescension, unlike, say, humiliation or scorn, lacks essential hotness. Assume that he is attracted to you at least in some part for your worldliness, and play it up. He is a tender, pink-eared schoolboy. You are Jeanne Moreau.
There is no instant technique applicable to premature ejaculation (and yes, 15 seconds is premature); it's all longer-term stuff. If interested, he can apply himself to his studies and gradually train himself out of coming so quickly, especially since it is likely nothing but nerves. Far simpler, though, is the magical solution available mostly to very young men and their partners: do it again. And again. And again.
Don't forget to read Andrea at Carnal Nation.com.