I think you already have some great advice, but I'm going to express my opinion as well.
Okay, he is going through puberty. He is going to be sexually curious. Be careful though. Don't isolate him because he needs to know that you care and you still love him. Going to the counselor is the right thing to do. If you need any more advice or just want a Christian perspective, contact Focus on the Family. They are great! They have a phone line and email just for counseling and talking to anyone who needs help of any kind. They can also tell you books that might help, and can give you ideas to help your son feel more comfortable talking to you about his curiousity.
I hope that he knows how blessed he is to have a mom that cares about him. My cousin was 13 when he got a girl pregnant with his first child, and his mom has made a lifestyle of not caring and letting him do whatever he wanted. He is unfortunate in that he doesn't have parents who care about his actions. I wish more moms of teens cared about the adult that child will grow up to be.
I have to agree with the one person who commented on the girls throwing themselves at him. Your daughter is at the perfect age to have friends that will get all starry eyed over a older guy. I did the same at that age. So talk to him about that as well. You probably need to talk to your daughter as well. I'm not saying he did this, but there is a small chance that he might have touched her or even just watched her. Just talk to her as well. One girl I went to school with got sexually curious at 8 and had sex with her 13 year old cousin. I'm not trying to make you scared or paranoid, but just prepared for the pressures that children and teens face today.
Well, I hope all goes well and that both your children grow up to be well adjusted, happy adults.