I know this must be incredibly difficult for you, my heart goes out to you, however I am unsure how will it help you and your son if someone comes on here and says, well yes this happened for a while with my son, and then it stopped, or conversely, now he's having x, y, or z problems?
What matters is your son's story and what has led him to this point, therefore please read the list below - it is taken from the Parents Protect website. They have a helpline too and I've linked to that further down.
It sounds like you were hoping the school would be able to explain where your son had gained his understanding of the mechanics of sex, perhaps from overhearing older children for example?
I sincerely hope that the staff you spoke to were not thinking your son is a pervert - I would hope their concern is the same as mine - which, as a parent and someone who has previously worked for years in children's social services, is that adult sexual knowledge in young children can be an indicator of abuse.
Equally it may not be (for example a child may walk in on their parents having sex, may find an older siblings magazine etc.), however I advise you to speak to a professional who can help you and your son through this so that you can determine how he gained this knowledge and then act accordingly.
If you do not want to call your local social services then the website I linked to above have a help line for advice and support.
I have included the 0 - 5 because he is only 6 and the ages are guides.
Pre-school children (0-5) years commonly:
Use childish ‘sexual’ language to talk about body parts
Ask how babies are made and where they come from
Touch or rub their own genitals
Show and look at private parts
They rarely:
Discuss sexual acts or use sexually explicit language
Have physical sexual contact with other children
Show adult-like sexual behaviour or knowledge
School-age children (6-12 years) commonly:
Ask questions about menstruation, pregnancy and other sexual behaviour
Experiment with other children, often during games, kissing, touching, showing and role playing e.g. mums and dads or doctors and nurses
Masturbate in private
They rarely:
Masturbate in public
Show adult like sexual behaviour or knowledge