My fiance has been diagnosed with moderate-sized suprapatellar joint effusion for over 4 months now and it won't go away, what could be the cause?

Injury vs over use: I'm assuming he is under a dr's care given the diagnosis. 4 months is protracted, if he is following his doc's care plan and not improving , a second option is indicated.

Usually trauma: Sometimes a person who has repetitive trauma to the knee will get pre-patellar bursitis. This is a painless, usually fluid filled swelling over the lower kneecap. People who get this include floor installers, plumbers, gardeners, and others who kneel down for a living. This condition is relieved by aspiration, removing fluid with a needle, and wearing knee pads to prevent abrasion of the knee.