Maybe he should wear a sign stating he is a covid survivor and immune.
Three things come to mind. I don't think the experts have had enough time to determine if one becomes immune after being infected and then recovering. There's also the question of if one does become immune, how long might the immunity last. Currently there are about a half-dozen strains of Covid and I don't think we know yet if one who has recovered might be immune to one strain and not the others.