Before people get too worked up about that and aircraft recirculating air etc, remember that the cabin air recirc filters are HEPA and equivalent to hospital operating standard for particulate filtering. Also air comes from overhead and out through the floor , so no fore aft air movement.
There is a genuine health concern for the few people around him and they should be quarantined, but not the whole aircraft.
Ps That’s for Airbus- can’t speak for Boeing. I do t know what type he flew on.