Well I'm glad that he told you.
I wish i had take the same action as him.
So a little about me, since i was 13 years old i was attracted to kids my age and younger all boys.
I knew i was gay but as soon as i got older and older and still feel attraction to younger kids, i saw was a problem, but i never told anyone about it just the fear to be rejected and the way people think about you, scared me so i hold my biggest depression secret. When i was 13 years old I found a lot of child pornography to download some I wish i never have watched, well i kept watching and download till 21 years old when finally the FBI got me. < this can sound bad but wasn't was such a relieve for me to get cached so i could get help, because there is no help, everybody that feel the same way I felt need to be cached to finally get help and that is sad because most people go to jail first and go with me i know people that went to jail because they download child pornography but never abused a children same situation as mine. I never went to jail that's because the FBI still working in my "CASE" , see I'm scared and every people that i know from my Group Therapy was scared of going to jail, I don't know how people feel about people like me, but going to a prison sounds horrible because you never got help to cure your self of this problem that you don't know how to solve and then you need to go to jail and be there with people that is worst then you, not judging because everybody got they problems but is scary to be there with murders.
So i live in US but im not from US either, I'm really glad that your son told you about what he feels. A lot of people don't tell their parents because they feel like me and all the other people that couldn't tell with the fear of been interpreted wrong because lets face it the world interpreters wrong, they don't care about you they want you to die or go to jail.
He is 15 so is still a good time to find a therapist to help him, look for a therapist that works with sexual offenders < that's what they call me but i hate this word. Why? Well no one helped me to fix my problem, then they call you that and put you in jail and the worst part is your name i'll be always dirty.
Not calling your son a sexual offender, he isn't one just like me. Why find a therapist that works with sexual offender? Because they will not judge your son like some other therapists does because they don't have experience with this type of subject.
I'm 22 years old now and if I was your sons age and my mom knew about my problem I would like my mom to be Open and understand that this that i feel i don't know how to fix it but I'm willing to try really hard to change (by the way now my mom know and my step dad and my sister) they are really been a support in my life. So just be patient with him and don't judge and remember that he told you something that is really hard to tell anyone.
Second part IMPORTANT!!
Make sure that he is not downloading child pornography / accessing any place that he can get images / videos of children in the age that he got a attraction to. Child pornography will not help in nothing just make the feeling worst and if you want to change you got commit yourself to not go to the place where is slippery.
Some things that you need to know of:
Depression / no friends / Fear to go to places like shopping mall thinking of Suicide That is what i felt most of my 13 to 18 years old life.
Depression: staying at home a lot eating, playing video games, surfing the web, not doing exercises, missing school. Not going to places where you can see another kids in both young and older ages.
Suicide all the time, I never tried to kill myself but i always though about that the world would be better if i killed myself. Or why god choose me to be like this and not someone else. Some suicide thoughts that i had like buying a gun and shooting my head, freeze to death, take some medicine like a lot till get a overdose, asking a friend to shoot me in the head, I never used drugs and i don't drink but i though of using drugs till i die or don't feel in my normal state of mind anymore, drown myself.
Well so be aware of that and help him out, just make sure that you are open to help him because this path is not easy but he still young so maybe tell him some stuff that i talked about here, so he can be aware of it too.
Someone edited my topic and removed everything that i talked about, for the people that is editing and trying to help like anongoodnurse if you could edit again, sorry I'm new to this place and by mistake I accepted someone else changes to my answer.
I would like to tell you one more thing just so you be aware of. Some people is recommending forums, I do not recommend any of them, from my own experience some of this forums there is a lot of people there that they don't want to change, some people may try to manipulate you to do bad things and don't let your son join any of this forums, if it is for your self so you can help a better way to help him its fine but if it is for him to join, just don't, there is a lot of people in these forum that is willing to help you, but there is a lot that just wants to do bad things. Even tho your son feel attracted to young kids a forum like that can be a way of other people try to manipulate your son. So just be careful. Virped i do not recommend, a lot of people there doesn't want to change(I tried to get help at this place and they banned me saying that i couldn't change) and lets face it the one with the name virped was really rude to you.
After 2 hours trying to find a article that i heard a while ago about a 19 years old pedophile that i feel exactly like him, i can post here to show you:
So it's a audio on sound cloud :
http://themamabeareffect.org/1/post/2014/04/a-message-from-a-19-year-old-pedophile.html
https://soundcloud.com/this-american-life/522-tarred-and-feathered-act-two-help-wanted < listen to this audio.
If you have any questions let me know. I'm glad to help people in the same situation and I hope some day we will have a better way to be helped without going trough the system.