There are indeed medical conditions which preclude the vaccine. But, just as a cautionary tale...
Some of those conditions are temporary, like, e.g., a series of cancer medication that comes to an end.
And some of the medical advice is ill-informed, even by a medical doctor; or, a lie made up by someone to avoid the vaccine.
My best friend's dad kept saying that his doctor told him not to get the vaccine because he once had a severe reaction to the flu shot. That seemed awfully suspicious to me since none of the ingredients in a flu shot that a person would react to are found in the COVID vaccine. So, I kept telling him to get a second opinion.
Well, finally, he messed up getting his 'story' (i.e., his *lie*) straight. The story morphed into a *nurse* telling him not to get the vaccine. And she did so before the vaccine was even available. Well, after his son just called him out for being a liar, and his doctor that was taking care of a shoulder surgery told him to get the vaccine, he eventually did.
Now, I'm not saying your brother's case isn't valid, it could well be. There are indeed medical reasons for not getting the vaccine.
But... I would encourage anyone being told they can't, to get a second opinion.
We still have medical personnel telling people they have to wait 2 weeks after a flu shot to get vaccinated, but that provision was removed by the CDC months ago.