Poor old guy. He shouldn't have to deal with this in his 90s. If I were nearby, I think I'd go over there and offer to help him guard his property.
BTW, is Missouri a castle doctrine state and do castle doctrine rules apply to someone who isn't necessarily guarding his own castle but someone else's?
It doesn’t really matter where you are in Missouri. If life or limb is in danger, you have no duty to retreat wherever you lawfully are.
This also applies to a 3rd party that you are trying to help - you can justifiably shoot to save someone else.