I know you are not concerned, but for the sake of other viewers that may be: The first thing to know is that in most cases, imaginary friends do not denote a problem with which a person should be concerned. As metacore points out, there are a couple of "types" of imaginary friends that can be cause for concern, but 65% of children seem to develop an imaginary friend at some point or another so if your child does have one, it is not the cause for concern some people believe it to be. It is also important to know:
Myth: Introverts are more apt to have imaginary friends.
Reality: Nope, extroverts are.
Okay, this myth was busted prior to the 2004 study. But it's worth
noting. In 1990, the authors of "The House of Make-Believe" — Yale's
Dorothy G. Singer, now a senior research scientist, and Jerome L.
Singer, now a professor emeritus of psychology — reported that their
research did not support the old axiom that imaginary friends are
linked to shyness or maladjustment. As Taylor puts it: "The kids who
are choosing to create imaginary friends tend to be more sociable and
to have more friends than other children."
It is interesting to know as well that most kids also know their friends aren't "real" but somehow manage to treat their friends with a very real sincerity. When I was about three I was very concerned because we left my "friend" Becky at my Dad's shop when we went to see him at his lunch time one day. Apparently, I through a fit because I expected my very pregnant mother to walk me back down to the shop to pick her up. Because I had clever parents, mom called the shop and my Dad devised a "fun field experience" day for Becky where she learned how to make whatever it was he was working on that day and I got past it.
My take? Sometimes it is just fun to imagine a world where your imaginary friend can swim to China and bring back a Panda for you to pet all in the span of a few minutes, or that your best friend is Disney Royalty and has invited you to attend the ball, or that your best friend keeps a tiger in his garage. I'll admit I do speak from experience here:
(I had many more imaginary friends than just Becky btw she is just the one that got left behind most often - probably my excuse to get back out of the house again on occasion knowing me) . . . In my case, and the case of my daughter when she had such a friend - it was someone to play school with - someone who could sit and read books as long as I could and who wanted to explore the woods while wearing a princess dress (because the boys are a lot of fun, but sometimes you have to have a girl along too) and from whom I could learn some things. My imaginary friend (Robyn) was much better at building forts in the woods than any of the boys I played with, my sister, myself or my real girl-friends Kirsten and Jess - I "learned" a lot about thatching and structural integrity from her :-)
Not all imaginary friends are human either. Another girl-friend of mine (this was around second grade I think), made her pencils into people by drawing faces on them and dressing them. She spoke to them and asked their advice on her school work (she now has a doctorate in astro physics). One of my imaginary friends was a pegasus that could hide in the clouds but allow me to hear whatever I wanted (I spied on politicians, teachers, anyone I thought might have something interesting or funny to say). Oh, and I met the Sandman too and he could help me travel to the land of fairy tales where I could go play in the woods with Red, blow over pig's houses with the wolf and even go on my own quests like St George or Robin Hood. (No, I'm not insane - just imaginative).
It seems my own hypotheses aren't all that far fetched according to recent studies
Myth: Imaginary friends are the ones who make mistakes, the ones
blamed for knocking over that vase.
Reality: They're often more likerole models — even idols.
"Kids are thinking about issues around competence — what they can and
can't do — and mulling that over in the context of interaction with
their imaginary friend," says Taylor. "Who can do somersaults? Who can
read? Who can tie their shoes? Who is riding a bike without training
wheels? A lot of times the friend will be the one who can do it all."
It sounds like your son's imaginary friend fits into this "role-model" category as his "brother" sounds highly successful. A healthy imaginary friend is like the characters we love the most in our favorite stories - they do things we might not be willing to do right away. We learn from their brave examples. They live in our imaginations when we need them for whatever reason - entertainment, living out our wildest dreams (whether they can eventually be lived out in real life or not) and yes, having the friend that will do the thing maybe even the real child isn't so sure he/she wants to do (a form of the child encouraging him or herself to take a healthy risk).
Myth: Big kids don't have imaginary companions. It's a little-kid
thing. Reality: Children can hang on to them long past preschool.
The assumption was that imaginary friends scram by the time their
conjurers turn 4. Wrong again. Taylor says one of the more startling
things to come out of her study is the stats on longevity. Older kids
(post-preschool, that is) continue to consort with imaginary friends
as they age. It's possible for made-up friends to stay a long time —
years, even. In fact, Taylor cites several famous adults who openly
admitted still having imaginary friends. Paul Taylor, a cultural icon
in the world of dance, attributed some of his work to his imaginary
friend, who he said was named George H. Tacet, Ph.D. (we're not
kidding). Call him the ultimate guest artist.
Often, it is simply one facet of imagination and one part of really coming to grips with fact vs. fiction and how far that "boundary" can be pushed - at least, that's my opinion.
You might find this article (from which the quotes come) interesting as well as this one which was originally linked in a comment. They are both about the same study, but interesting none-the-less.
In fact, according to Bob Trapani, owner of Thrive by Five and chair of the New York State Occupational Therapist's Association, you might want to not only not discourage, but even encourage this imaginary play.
So instead of worrying about an imaginary relationship your child has,
think of the rewards they are getting from it and help them reap the
benefits by welcoming their friend into your life.
For example, invite her (or him, or it) to join you for meals, outings
or other family activities, and follow your child's lead. If they want
their "friend" to have a place at the table, help them set one, if
asked. But don't use your child's friend to try to “change” your
child’s behavior ("Look how 'Scooter' ate all of their lunch! Why
don't you?"). In addition, don’t let your child use their friend to
escape consequences (“'Scooter' didn’t break the lamp, you did").
Lastly, remember to provide your child with plenty of other real-life
companions and opportunities for other kinds of imaginative play. To
quote noted author and artist Barbara Goldstein, “An imaginary friend
is often what the child needs it to be.”