Questions that come to mind . . . what is his history with getting the flu? How did he socialize in general pre-pandemic? A lot, a little, etc. Is he a "hugger" who tends to get very close to people on a regular basis, especially family and close friends.
Like a flu shot, a COVID-19 vaccination is only one precaution that makes it less likely that someone will get infected. Their behavior around other people is a major factor as well.
I had quite a few years when I was working that for one reason or another I didn't get a flu shot. I didn't get the flu. But my personality is such that I tended to avoid shaking hands even for some professional meetings with clients of my company. In general, I tend not to get very close physically to anyone. That includes family members.
Omicron changed the situation. It's more infectious but causes less serious illness even among unvaccinated people. Although at this stage someone who is unvaccinated may well have had a case of COVID-19 and not know it. So could be that they already had some level of natural immunity.