Prostatitis causes?

Two months ago my husband noticed a brown discharge coming from his penis and shortly after developed symptoms of burning while urinating, frequent urination, constant need to pee but little came out. The doctor screened him for Chlamydia, gohnorhea, HIV, and uti bacteria but they all came back negative. He did two weeks of cipro, the urinary tract symptoms got better but didn't go away completely so they did a prostate exam. His prostate was swollen so they continued him on cipro for another three weeks. His symptoms stayed the same. Pressure in his rectum and the tip of his penis was sore with some occasional trouble urinating. They switched him to 2000mg a day of penicillin and it's been a week and his symptoms still aren't better. What could he have? When his STD tests came back negative we had sex, should I be getting tested for anything?  Could an STD be causing his symptoms?  Thank you for any advice you could give.