It sounds like a mental health issue to me, especially given his previous way of being - as you have described.
Most people can express an opinion either way without falling out with best friends/their partner.
It’s a tough one. Don’t want to bang on about my relationship, but my first serious boyfriend (from 16- 25) ‘became’ schizophrenic 3 years into our relationship following my dads death (trauma) and his twin was showing signs up to a year before following a relationship breakdown (trauma). They both had some strong beliefs about things that just were not true (eg conspiracies to kill them), but these intertwined with things that were more subtle- and were true.
The latter beliefs becoming true, fed their belief in themselves and their world view and cemented their belief that they were right about it all.
I left at 25. My previous partner was, long before then, unable to work/ support himself, on meds and had a psychiatrist and social worker- this had been the case for a couple of years and there has been no improvement.
My point being a) a traumatic event (covid/lockdown etc) can trigger a predisposition to mental health breakdowns and b) it is extremely difficult to reset these false beliefs which are set in fear- when chances are there will be an element of truth, which then feeds the fear - but also trying to reset their thinking just ‘proves’ (in the paranoid mind) that you are working ‘for the enemy’ and ‘against them’.
I’m not sure, having witnessed these 2 cases, that I believe in a cure 😞 .
Of course, in your personal case, it could be just a misguided opinion that is all encompassing now (as covid dominates life) but might be irrelevant in a year or two when normality presides. In which case maybe it’s something that you can just agree to disagree on? I.e you don’t engage and if you choose to get the jab you don’t mention it.
I think this is possible as aside from this you describe a seemingly ‘neuro typical’ person. Had the issue been something that will forever exist (eg the sun is spying on me) then I would doubt the chance of moving past.
I really hope you come out the other side together happily.