My boyfriend is experiencing lower left back pain close to hip noticably swollen to touch and to eye . no other symptom.

Xray: I would maybe suggest he go be seen by his/a doctor and have an X-ray of the hip and rule out a septic joint. You don't mention him having a fever which would make me think that even more, but could be early in the game. Still with pain and obviously swollen back/hip I think exam and X-ray are warranted at this point. Best wishes to you both.