Other times I simply go along with whatever he is saying or cave to what he wants me to do, which has led to resentment over time as well as me making decisions that are not in my (or his) best interest.
you might think of this as "validating the invalid". in general, you dont want to fall into that trap.
the short answer is that there arent really tools to address paranoia, if by that you mean, assuage it, or fix it.
this is a part of him, and tools for coping are really the heart of the matter.
this is on a much smaller scale of course, but have you ever met some people that just love to complain? think of this as, kind of, a form of that. and then the question becomes how you deal with it.
sure...if you love him, you dont shut him down. you listen, within reason. let him talk, actively listen. paraphrase, ask questions.
it sounds though, at least a certain point, that you really arent comfortable with these conversations. thats key. it may be as simple as saying that, agreeing not to discuss certain things, like some families at thanksgiving avoid talking about politics. youll need to tread lightly if you choose that approach. it will not be as simple as coming out and saying youre uncomfortable with it...he will, like anyone would, experience that as rejection.
and if you see that discussing things just gets him further riled up, it may mean knowing when and how to disengage and exit the conversation artfully. i have a family member that is prone to talking my head off. saying too much or too little tends to just encourage them to talk more. sometimes i have to let them know i have to tend to something else, go to the bathroom, whatever. sometimes they dont even hear me and carry on, sometimes they get mad...anticipate this.
its a balance. in any or all of these approaches, personalize them in terms of what works uniquely between the two of you. it may take some trial and error.