Please look into buying him some books so he can understand what his body is going through and about to go through. He needs to know that you may not have the words but you will help him learn to know what he needs to know. Then give him time and space to read those resources. There are some wonderful books that handle the subject of bodies changing delicately. This is a life moment. Give him the tools to be empowered by knowing facts. Let him have the books on his shelf so when he feels like thinking about these topics he can get answers by himself for himself. Also once you give the books you can talk about the subjects as questions arise and make sure you let him know that no question is wrong, it may make you both uncomfortable but this is a life lesson on communication.
A quick scan of Amazon reminded me of these books that are excellent resources:
It's Perfectly Normal: Changing Bodies, Growing Up, Sex, and Sexual Health (The Family Library) by Robie H. Harris and Michael Emberley
What's Going on Down There?: Answers to Questions Boys Find Hard to Ask by Karen Gravelle, Nick Castro, Chava Castro, and Robert Leighton
American Medical Association Boy's Guide to Becoming a Teen
My Body, My Self for Boys, Revised Third Edition (What's Happening to My Body?) by Lynda Madaras and Area Madaras
What's the Big Secret?: Talking about Sex with Girls and Boys by Laurie Krasny Brown and Marc Brown
What's Happening to Me?: Boys Edition by Alex Firth, Susan Meredith, and Adam Larkum
"What's Happening to Me?" A guide to puberty by Peter Mayle and Arthur Robins --- My personal favorite from when I was a kid
http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3...
http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3...
Good Luck.