I caught my 6yr old son playing with his willy?

I'm wondering why the house is so quiet, why is my son not up under me like usual. I walk past his room thinking ok he's probably watching something really good or sleeping. I see that he is under his blankets, I pull the blankets back he naked with a woody. So I told him to stop, get dressed, and lets talk. We went over a lot of important issues about touching, because he does go to school, and I like to make sure my kids are alert. Everything is fine, so I asked him why did he do what he did. He said because it tickles him. So I said that's nasty and to try and not do that again. I called my mother who raised 3 boys already, and she said to just ignore it and he will stop. Is she right, she always is, but he just grossed me out. He was in his bed, in his room but he's 6. I thought that I would at least have until he's a teen to have to deal with this.