Not to worry. I had (at the time) a 5 year old cousin that stayed with us for awhile was very proud of his little erection and showed it off to me and my friends. We would look at it and just giggle and laugh and thought that was cute but he has obviously grown out of that.
It is normal. My son went through a stage like that, but he was actually playing with himself. I was so stressed out, I even took him to the Doctor, but they said that is normal.
Yes, it's normal. I overheard one of my son's friends saying Jessica Rabbit's voice makes his penis "long" at that age.
What he needs to know is he needs to not share this information - it's normal, but it's private for himself to know.