J.R.
R.,
I don't know if your sons imaginative behavior is normal... but, I have a story to share...
My daughter was about 2-3 when it started (she's now 10) - she would play for HOURS with her imaginary friend she called Mimi's Papa. My mom was called mimi by all her grandkids - so my translation was my mother's father. She described mimi's papa to us and he sounded a lot like my mom's dad - physically and in some manneurisms (my grandfather would say 'oh' alot in response to a statement...
anyway, she concocted these wild stories of things her and mimi's papa would do wit hher... they played with wolves in a cave, and he bought her a motorcycle, and she would tell everybody about him, as if he was real. to her, he was real.
it did go away - and she is a child who very much enjoys her own company... she can stay on task with whatever she is enjoying for hours.
You may want to consult a child psychologist to see if his behavior is not normal or out of the range of mental development... sometimes ped's too easily dismiss things in my opinion... and considering they see so many patients (how long do your appointments last? my last appt with my 2 mo. old was about all of 7 minutes of her time)i'm not sure they have enough info to make an educated call on these things.
Good luck to you.