K.E.
I had to smile when I read this. I am a huge fan of Ghost Hunters and had a moment of panic when my daughter started having conversations with Mimi at age 3. In the end Mimi and soon, Hand were her imaginary friends. Its normal for a lot of kids to have them so I wouldn't worry too much. That being said I had a family of ghosts watching out for me, but they came into being after I had a strange experience at 5. To this day I will swear that my Grandma came to help me when I was scared. So while Girl Ghost was very real to me the rest of the family was imaginary.
Just keep an eye out for him. If he starts having a lot of nightmares or saying/doing things way out of the norm, you might want to question him a bit more. If he is in school he may be hearing stuff from other kids and doesn't want to get them in trouble. Imaginary friends are good to try new things with or deal with new situations. This isn't unusual for kids, so just take it in stride and see if anything develops. Best of wishes.