6 1/2 year old son caught touching neighbors penis

I just received a phone call from my neighbor saying that my son put his hands down her sons pants, touched his penis and told him not to tell his parents.
I was overcome with sickness! I hung up the phone and asked my son about what happened at his friends house.
He told me everything. They were playing a game, he touched his penis, told him not to tell, then asked to go home. I asked him why he asked to go home and he said because he knew what he did was wrong and was upset.
What worried me most was the fact that he told the other boy "don't tell your mom or dad"
This got me thinking. I asked my son very calmly if HE has ever been touched inappropriately at any time by anyone and told the same thing (don't tell mom or dad) He denied anyone ever doing that to him. I asked him what made him think he could touch his friends penis, he said he didn't know, but he knew that it was wrong. He was very emberassed!!!
This same boy has had his penis touched before at school. I'm wondering why kids are wanting to touch this kids penis?
Either way, I told my son that that type of behaviour is not acceptable. That we do not touch other peoples privates, nor do we allow others to touch ours.
We finished our talk, gave eachother a hug, I told him how much I love him and then it was over.
He went on to play with his brother like nothing had ever happened.
what should I do or should have done?