I can tell you that I know this feeling all to well.  My son who is now 16 years old and started doing this at an early age.  I was so upset when I found out that I cried too, feeling like a was failing as a parent.  You see I was brought up that things like this were just not something you talked about in the home.  Today, it seems that it is important to have this discussion even with our children at a young age.  They are even learning things in school that we did not even know until we were alot older.  I can tell you from experience that I had to seek God on this matter because once I found out I started looking and observing things all around me.  One night I was awaken (I know that it was God) at two in the morning and something told me to get up, I wanted so bad to go back to sleep, so I turned over and I heard it again "Get up"  exhausted I said ok Lord let me know ....again "Get up" so I crawled out of bed and went into the kitchen where we had our computer hooked up and my son (13 at the time) was looking at pornography.  I was shocked and horrified.  I cried and I hurt.  My son looked at me with such shame on his face.  Not the look of  "oh no I got caught" but the look of total shame.  The next day I had my internet turned off.  That night I found a note on my pillow it was from my son... he told me he was so so sorry for what he done and he cried out to me in that letter asking me to please pray for him he did not want to feel this way anymore.  It stolen my sons innocence at such a young age.  I got down on my knees with my husband and we prayed.  I went to my son the next day and I sat down and talk to him about what he was going through.  I let him open up completely to me. 

That next week I went to the christian book store just looking for music cds and on the shelf was a book "Every Young Man's Battle" I opened it up and low and behold there was a full section on this very subject.  I purchased the book and gave it to my son who read it immediatly.  I have also read some of this book and I want you to know it is never too early for you too talk to your child.  I encourage you to read this book maybe it will help you how to approach your son.  Know this .... your child needs you now more than ever. It does stir mixed emotions up inside.  I will be praying for you that God will guide you in this matter.