I'm still new'ish to the board and it's the first time that I respond to "bad news". I never really know what to say without using expletives about this terrible disease. I'm sorry that you have to go through this.
However, I agree with the general sentiment here, 'it's not over until it's over'.
In Germany a company developed a new peri met treatment called PIPAC - Pressurized IntraPeritoneal Aerosol Chemotherapy. The main research institute is the Ruhr University in a city called Bochum. Apparently the side effects are less severe than with HIPEC. I looked it up and the University of Texas used to be a research collaborator. Head to Texas Endosurgery Institute, San Antonio http://www.texasendosurgery.com
Here is a manufacturer/developer link in English: http://www.capnomed.de/en/treatment.html
I'm sure you looked in every direction but I agree with plastikos that now is a good time to pull out all stops. Hyperthermia is increasingly becoming an add-on standard in Germany; same for high-dose mistletoe infusions. I can PM you details of clinics in Germany where they include this on top of chemotherapy (and which are not cookook lunatics who promise to cure all cancers if you give them all your money). While it will probably not cure your dad it may give him time until you come up with another plan of attack - the exact reason why I just bought an infrared sauna (cheap second-hand off ebay).
I hope you can muster up the strength to keep looking until you really, really, really have exhausted all options. Lots of strength to you and your family.