My 84-year-old father is still trying to work, but it's getting unsafe for him and others. How can we get him to quit and tell him he has to stay home?

Yes, I would suggest volunteering, staying active, working at something else. Get him signed up for public transportation (many places have it for seniors with smaller buses and more informed bus drivers who Help them decide if this is their stop, etc)
Because if you Force him to Stay Home he will hate it and then you don't know how quickly he can decline if he gives up on life.
I would recommend a senior center, if there's one anyplace nearby. Or a VFW or some kind of group that has seniors in it, a place where he fits in, and can find friends. We all need a support group at any age, a peer group we can relate to, you know?
At least you Care that he's ok and happy with how he spends his time. Tell him how very much you love him, be gentle with Dad.

Is it possible for him to work part time? volunteer? or get a different job? I know that when I become a senior, the idea of staying home would not appeal to me, especially because I have always worked.
What do you mean "unsafe for him and others?" Does he have a cognitive problem?
Do your best to find something meaningful for him to do that is also safe so he has something to transition to. It will also help keep him mentally healthy.
good luck