Should my dad still be driving?

I am so worried about my Dad still driving aged 89. He is of sound mind, but has dreadful arthritis is his legs and not a lot of strength in his arms, so I don't think he would cope in any sort of emergency stop. He is an anxious driver anyway, and doesn't seem to have any awareness of what's going on around him on the road. He is encouraged to drive by my also 89 yr old step mother to keep their independence. There are a lot of family members who can take them shopping etc, so there is no problem there. My dilemma is how to stop him driving, we have all asked him to stop - but he won't. I think there is big problem with the system for elderly drivers, they just have to tick a few boxes on a form - and then they keep driving. I really think there should be a stricter control on this. Any one else had this situation - any help would be great