What Causes Knee Swelling?

Hi my husband have a swollen knee at the moment and when you touch it feel like touching silicon. His GP doesn t what cause it hence he is not on any medication. He is in pain and walking is very difficult. Also lately he pee a lot, day and night and when he does go and pee...empty. He was sent to have an ultrasound done and the result a 4mm kidney stone. Still is the frequency of wanting to go pee is still there. This have been going on at the start of 2015. Early this year after getting a routine blood test, he was diagnose with HLA-B27. He waiting to get an appointment with the specialist to get that check out. My husband just turned 40 years of age. Could you please share your thought of what might be wrong with him.