You can try and persuade your dad without relating it to his cancer in any way. Set it in more general terms as something it is important to do especially with nasties like Covid, flu etc around. Really, both your parents should sort their affairs out not just your dad.  Also, are your own affairs in order? How general are you willing to make any discussions? Good luck!

Btw I just looked back over your earlier posts and saw that your dad is 80. Wow! People are generally advised to sort out their affairs so don't leave big mess and muddle a lot earlier in life. If he hasn't sorted things out by now, sadly he may never do so. This isn't really a cancer issue. It's bigger than that. It's going to be very hard to get everything done now given his age and what the doctors have told you in terms of prognosis. Tricky! x