He is experiencing discharge mild irritation during urination from penis sometimes itchy. he got tested everything was negative. what could it be?

Urinary discomfort: i need to know what kind of testing he had done. If it was a simple urine analysis then it is not an exact test. Yes he may not have a UTI but he could have a inflammation of the urthera, a std, or prostatitis from a std. He can take cranberry juice but a small dose of abx help but if it is a std then he will need different Abx. Next would be he need a uroligist.

Urethritis: Virus can cause dischage. I'm a little suspicious of the testing there given the clinical findings. This sounds more than just a UTI.