It's so frustrating when doctors say "there's nothing wrong". A more correct statement would be "we don't know what's wrong".
If a child who previously didn't complain of back pain and who seems normal otherwise suddenly starts complaining their back hurts, it's because it DOES hurt!
Can you give him tylenol occasionally? Sounds like it may be a muscle pull or something that will heal eventually.
maybe you should relax his muscles since you said he plays sports a lot its common to have problems like those and it's best not to over do it in sports because lately their's this problem with kids injuring themselves from over sporting
Is it upper back or lower back? Over the spine or on the side of the spine (large muscle groups on either side of the spine on the upper back). Any signs of bruising?