Excuse my language, but OMG! Is his hip appliance made with titanium? As a sufferer of Allergic Contact Dermatitis, ACD I do know that those of us with allergies never adjust to what our bodies believe is a toxin, for him it is nickle and titanium. It seems the brutal reality is that it's gotta go. I do know personally about an allergy to a substance, not just affecting the area of contact, but raising inflammation so the entire body will be affected. The longer he is in contact with his allergy the more his overall health will deteriorate. I am so very very sorry to even write to you about this. It breaks my heart.
Yet lets look at this as his glass being half full, if his appliance does contain titanium you now have identified the cause of his heath issues (no small task)! He can regain his overall health if it is removed. Even removed although he will feel better within a week, it may take up to 6 months for this ACD autoimmune disease to calm down to normal. So keep that in mind, as that happened in my life even after I knew what to avoid, because the inflammation that is causing his rashes takes time to settle down. I have no idea about a solution for a replacement part for what he has now, but his overall health, even if he is limited to a wheel chair will improve and he could have a good quality of life.