Someday my daughter will ask the same question you have.  right now I'm lucky she thinks I'm just lazy.  If your father is like me, he is very aware of his limitations, and his actions should be accepted.  However, we can be tricked into doing things.  Try wording your offer to do something differently.  Some ways of asking that may ellicit a more positive response are "I'm going to .... wanna tag along" (this may make him feel more protected because he doesn't have to assume a leadership role in the expedition, he's just 'along for the ride')  or start small "lets go get a drink (take him to whatever gas station he trusts for a soda)  then continue "can we run to the .....  I need to grab something..."  (If he's suffering from agoraphobia you are slowly conditioning him towards targets farther from the norm, it may take an hour to get to your actual destination, but he isn't jolted into it.)   If it is a day where he has showered and has clean clothes, you are much more likely to get him out the door.  Did he sleep well the night before?  Big issue!  If he didn't, he won't feel like he has the mental energy to combat whats taking place behind the voyage, and he won't risk it.  Of course I could be totally wrong--these are just things that work for me.