My 11 year old is complaining that his left testicle hurts?

Go to ER now: This can indicate a very serious condition called testicular torsion that needs to be evaluated immediately - go to your nearest emergency room now.

Left testicle: pain can be due to infection or torsion or tumor

See urologist...: He needs to be checked out if he got torsion of intrascrotal apendages (of epididymis or testis), torsion of testis, UTI-related epididymitis, etc. So, collect the onset, degree, duration, interval, and possible associated symptoms of testicular pain over time, and bring those to urologist for analysis, more history, physical exam, possible tests so to deduce possible diagnosis for proposing care.

Get checked: If the pain is sudden, intense or worsening he needs to be checked right away. This could be a torsion which can result in loss of the testicle. Also could be infection or an injury, but can't tell without getting checked. Good luck!

Testicular pain: If the pain is mild, it's probably nothing. However, if the pain is severe, persists or his scrotum is swollen; it could be testicular torsion. If you suspect that, he should be seen immediately.