DD7 and DD9 just recently decided to create some new imaginary friends. They discussed it, determined names, sex, age, etc. and started playing. We have always had imaginary pets, people, things over the years but I sort of thought maybe we were done with that.
They like to name everything in their rooms (sometimes with tiny scraps of paper taped to each thing) and stuffed animals, legos, Barbie's, etc. play out all sorts of roles and scenes.
They used to come up with wild, creative situations all the time no matter if we were in the grocery store, park, or at home - it didn't matter. However, I think at the age of your DS (5) it was more common for them than it is now at 7 and 9, so he may start to grow out of it soon. You didn't mention it, but is he an only child? If so, then I think that might make even more sense, for him to be so over-imaginative.
When DD7 was around 4 years old her pre-school teacher asked to talk to me one day. She asked if I just had the 2 girls or if I also had a son or maybe a nephew was living with us or something. I sort of laughed it off and told her I just have 2 girls and asked why she was wanting to know. She told me that DD had talked so much about her older brother in vivid detail that she thought I had an older son even though she was pretty sure I just had 2 girls. We laughed and laughed and she gave me some examples of what DD had been telling everyone. DD had him down so good in her mind that she never messed up in telling about him, so that made it seem to the teacher like he must be real. I later asked DD how her older brother was doing and she got such a sweet, impish smirk and proceeded to tell me all about him. I asked her how that got started and she told me she and her older sister were playing make believe one day with an older brother, so she just carried it over into school.
I'd say what is in our kids' heads is so much more exciting than real life that it can be hard for them.
IMHO - I have always told my girls if there is a book that has been made into a movie - they have to read the book first before watching the movie. What they can imagine while reading a book will be much better than the movie and if they watch the movie first it could mess with how they picture things in their head when they read the book. Now that they are older and have had a chance to read some books that are also movies, they agree with me. They have even made comments about how the movie director really messed certain things up or that wasn't how they pictured it when they read the book.
