This is incredibly late response, but in case anyone else with a similar problem reads this maybe it can help them. I have an extensive background in child development and psychology. I've worked with hundreds of children.
Yes it IS very common for boys between 5-7 to experience erections for the first time. Generally they are spontaneous and uncontrolled. Many boys experiencing this can be somewhat concerned, frightened, or just claim the erection "hurts" or feels uncomfortable. It's all part of the body preparing for puberty. Many children, male and female, "masturbate" as young as 2 years old. It's all part of them exploring and discovering their body.
In your case, it is very possible that your some may have been accidentally exposed to pornography, or something similar on television. Many children walk in on parents in the midst of sexual acts sometimes without the parents even realizing the child has seen them. Another theory is that another child, possibly older shared information with him, such as what oral sex is.
You might suspects abuse if your child has other serious symptoms or behavioral issues. Depression, anxiety, fear of adults etc.
If sexual abuse is ruled out, simply talk to your child. Ask them gently "where did you learn about these things" and let them know, their privates should not be touched or played with by other people and it's ok to say "no" if someone asks. Emphasize that your child's probated are for them and them only.
To discourage exposure and masterbation type behaviors tell them "our privates need to stay in our clothes unless going potty" and or "if we've been touching our privates we need to go wash our hands so we don't spread germs" Be gentle, understanding but very consistent so your child understands appropriate behavior. Avoid shaming or punishing your child as they do not yet understand privates uses and what's appropriate and could lead to self-shame, embarrassment, and sexual confusion.
Also remind your child that sexual acts between people is for grown ups only.
If things progress rapidly or you notice other concerning behaviors find a wonderful therapist who is experienced in handling such cases.
Most importantly, show your child unconditional love, compassion, and support as they go through the changing stages of their body.
Its coming from somewhere, someone, or something he observed! 6 year olds typically don't engage in these sexual behaviors. I really don't want to second guess what is going on. That is why I am pointing you and your son in the direction of mental health counseling so that you can get to the bottom of what is going on.
If these behaviors occur at school, you may have bigger problems. That is why it is best to get this behavior evaluated now.
My son an I are Very close. He's Just starting school this year, for the first time. Up until now, he's NoT been away from Me. With the exception of Family, close family. Ann, NeVER over night or anything even with that. As I said in my post, I have TRiED talking to him, an asked repeatedly all the "has anyone ever" questions. You really think this is coming from sexual abuse? Ugghhhhhh, Im FREAKiNG out right now.
Yes. Take him to see a counselor ASAP. It is unusual for a 6 year old to be this sexualized. Sexual abuse needs to be ruled out. Seeing a therapist may be the only way to rule out sexual abuse.