Thanks for writing in with this interesting question. No two children are alike and as much as we try to give guidelines in psychology about what behavior is “normal” versus “abnormal,” they are still just guidelines. Yes, his interest in sex seems to be a little advanced for his age, but not dramatically so. I also think that kids are getting exposed to things much earlier these days due to TV, the Internet, social media and peer interactions.
I don’t necessarily feel that you need to let him know that you read his journal, but I would continue to have very open and direct talks with him about sexual things. With his curiosity, you want to make sure that he is getting the right information from the right people: you and your husband. At a minimum you might check out some books from your local library about child sexual development and how to talk to children about sex. It might also be helpful for him to read some age appropriate books as well and then allow him to ask questions afterward. If the concerning behaviors continue or you just start feeling like you are in over your head, then it would be a good time to seek the advice of a counselor with experience in this area.
All the best,
Dr. Holly Counts