How do I keep my father's condom catheter from coming off?

Since this is a recent problem and you have had no other problems in the past I would guess this is a manufacturing problem and I am sure the company would want to know. Often if they get a few complaints with the same problem they may recall all. You may get this order replaced free of charge. It is worth a few minutes to contact Customer Service.

As a disclaimer, I'd say your best & safest bet is to get a new box.
Otherwise, you can try to makeshift something with medical tape to tighten it or attach some of the adhesive to part of the shaft and on part of the catheter (assuming the condom part doesn't reach too far). Maybe its an issue of the adhesive not sticking properly (if the ones you have use adhesive) and cleaning could help. If it's an adhesive+size issue then making a loop of double sided medical tape, placing that first on the shaft, and then putting on the catheter?
Maybe an extra large condom with a hole cut in it and slide that over the whole shebang?
Lmk if you have more info about why it's falling off and I might have some more precise ideas! Best of luck.

Triplicate actually.