The prostate would be the first thing that comes to mind. I would assume that basic tests like a urinalysis and tests for gonorrhea and chlamydia have already been performed.
You can consider a prostate exam to evaluate for prostatitis, as well as sending off any prostatic fluid off for analysis and culture.
A transrectal ultrasound can be done to evaluate for any abscesses.
There are some cases of prostatitis that require an extended course of antibiotics.
You can discuss these options with your personal physician.
Followup with your personal physician is essential.
This answer is not intended as and does not substitute for medical advice - the information presented is for patient education only. Please see your personal physician for further evaluation of your individual case.
Kevin, M.D.
http://www.straightfromthedoc.com