Straight to his face.
We actually have a really great relationship. The rest of the office is totally brainwashed about vaccines and other leftist bullshit, so I'm the only one who is openly refusing to get vaccinated. He came to my desk to tell me about how Walgreens is offering vaccines to basically anyone and I said "that's great news for people who want the vaccine." He asked me would I really not get it? "Nope." Why? "Because I don't like it, I don't think it's necessary, it's an experimental injection and not even technically a vaccine, I believe in my immune system and I don't like how it's been socially engineered and forced upon the public. I don't like it."
He was silent.
I continued with "and so if you decide vaccines are mandatory, I'll be working from home." He said, "well, if vaccination was mandatory and you refused, you'd have to find another job." I said, "yep."
"You'd find another job?"
"Yes, I would." I was on my way to deliver something to one of my interns on the other side of the office, so that's where the conversation ended.
I’ll put it this way, my preparedness to leave if it is mandated is a real consideration for him that I know he would seriously consider. I’m very valuable to the company.
The interesting thing would be to find out who is advising him.
Many companies are taking the advice of attorneys, and most attorneys out there have NO BALLS when it comes to rocking the "mainstream" boat on anything.
They are advising people to "be on the safe side and force masks, etc." when in fact they are actually advising people to engage in a felony -- conspiracy against rights.
Most attorneys these days are like most doctors these days -- they learn falsehoods in school, and then presume they are smarter than everyone else, while they bend over and do what they are told rather than think for themselves.
They pass on their false indoctrination to the clients and patients. Law schools and med schools are both corrupt in a corrupt system.
If I were in your shoes, and had a good relationship, and though I could ask tough questions (depends on his personality and your relationship, etc.), I would probably ask him where he is getting his poor advice from.
I would ask him if some attorney is advising him on this. Or is it a doctor? Or his wife? Or CNN?
I would also ask him why anyone would be advising him to do harm to his employees by forcing an experimental cocktail, when there is no evidence at all that it is effective or safe.
I would make him THINK before acting.
I get it, and it sounds like you unofficially told him to stick his job up his arse if he tries that stunt. Kudos.