You can turn on a feature on Android in which you can say answer or reject when a call is coming in. If you do that, I believe when the call gets answered it automatically goes to speaker phone. You can also turn on a feature where pressing the power button will end a call and pressing the home button will answer a call.
These features are going to limit him though, because if he is using voice command (Google Assistant) only he won't be able to do things like press buttons for making calls to automated systems.
If he can't see well enough to hit these buttons, he really needs to turn on either TalkBack or Voice Assistant. Both of these are screen readers that read everything on the screen.
Would you mind sharing other things you have done with him to help him with his phone? I'm in this exact situation with a client who doesn't want to use TalkBack and am stuck how to help him with automated systems.