It is the first time that I have ever heard about a child as young as six years trying to stimulate another. I cannot say that it is normal. Some suggestions:
1. Tell your son to refuse to be handled and to push away anyone who tries to touch him sexually. And he should scream for help. This will be useful advice for any future sexual abuse.
2. On TV and the Internet, so much explicit sex is constantly available. You will have to keep a watch on what shows the children are seeing.
3. Recently, there has been a worldwide discussion about and celebration of homosexuality. It is possible that somebody told the 6-years-old child that it meant two boys having sex.
4. Instead of placing the blame on any person, I think both sets of parents should meet the two children together and tell them that such behaviour is strictly a NO. Tell them that it could lead to many problems later. That if they say sorry and stop the sex, you will not bring it up again or tell anyone. But if it is repeated, you will have to send them to a Psychiatrist for advice.
5. You could talk to a Child Psychologist in your city and find out what advice is forthcoming.