Just make sure he knows the behavior is wrong, and if it continues, take him to see a psychologist.
The software blocked out the verb you used. Depending on what he did, it might be something to worry about or not anything much to worry about. For example, if the word was "stomps" that would mean something different than if the word was "ejaculates." (Although I can't imagine a 6-year-old being able to do that.)
If it was something overtly sexual, then you have to wonder where he got it. For example, if it was s u c k ing then you should look to the people in the 6-year-old's life and ask him if he had this happen to him from someone else. I don't *think* (although some people might say I am mistaken) that doing something like that is considered in the normal range for kids who are just playing, it seems like it is too much a learned behavior when the kid is not through puberty yet. Do you think there is any way someone in his life might have done this to the 6-year-old?