A good friend suffered with it for several years, in his case it was due to an undiagnosed genetic defect which is quite rare. Acute phases are as PPs have said, managed in hospital with fasting/ pain relief. I don't know much about the diet as my friend was a bit naughty and just tended to avoid eating as much as possible. Of course that lead to breaking out and eating heaps which usually lead to an attack.
It is excruciatingly painful and I have heard it is one of the most painful conditions you can have. for you and your poor dad. If your dad is in hospital they will be looking after him and telling him how he can manage it. In my friend's case he hasn't had an attack in about three years since they were able to do something for him that resolved the issue somewhat. Hopefully there is a solution for your dad's problem.