I don't know a lot about hip replacement but my c-section was horribly infected about 10 months ago and I was in the hospital with IV antibiotics for 12 days. I was told it was a bacteria that is normally on the skin that got in the incision (not even very deep but the infection generated a lot of infected fluid that had to be drained). During that time I was on about 5 different antibiotics and/or antifungals (first IV and then orally). I would definitely request a consult from an infectious disease specialist. If they identified the bacteria then they should know what to use I would guess. I also took a lot of vitamins and probiotics after the infection including the vitamin D that was mentioned in another post
On another note, having a serious infection and especially a recurring one, is very scary (a wake up call about one's own mortality) as well as being really painful (my infection was much more painful than the labor or c-section). Try to be as patient and reassuring as you can. Also making sue he has plenty for friends and relatives visiting/calling may help him keep in better spirits.