Hi,
i've heard that bladder infections are pretty common among children though I never had one when I was a child. I just spent the last 2 years going to doctors and they told me nothing was wrong and it's all in my head. Turns out I have a horrible bladder infection (ureaplasma urealyticum) and it didn't turn up on the routine culture tests.
What i'm saying is just because the doctor says "there isn't anything wrong" doesn't mean there isn't anything wrong.
I hope your son starts feeling better soon.
Take care