Just wait until he sneezes and you say "bless you."
It's a cult. They're conditioned to strongly reject very common things. Phrases like good luck and bless you. Objects like crosses and steeples. Personal choices like beards, long hair on men, piercings, etc. These are all things that they've been conditioned to recognize as "worldly" which, in turn, is recognized as being evil, lying in the hand of satan, and a part of satan's wicked system of things. This is designed such that outsiders can't help but give themselves away as being an outsider in a way that any JW will be all too aware of. This builds walls between the JW and the non-JW to prevent them from developing relationships outside the cult that might undermine the cult's control and leave them with a system of support should they ever consider leaving.