Very difficult. I always contemplated giving up my driving licence when reaching 80, but as Providence stepped in and rendered me permanently unable to drive safely (TIA resulting in permanent sight problem) a month or so before the fateful day, the need to make the decision never arose. Had the occasion occurred I have some doubts as to whether I would have followed my intentions, my driving seemed OK, and I deliberately made allowances for slower reaction times (but not covering emergencies obviously). I had considered the IAM review and would possibly have gone ahead, but now I can't of course. And my Neurosurgeon stressed the permanent nature of the ban, so no redemption likely unfortunately.
As to the answer to your problem, I suggest regular "nibbling away" at the issue and it may begin to sink in. It is very hard to give up driving - even in my case where I had good intentions, I was actually devastated when it was forced upon me. But I've come to terms with it now, "it is what it is".
Can you make any helpful overtures to your father as to how you could soften the blow for him (without offering an on-demand taxi service of course).
P.S. Sorry for rambling on a bit about me me me, but I thought it might give some insight as to what your father is facing.