I got my terrier when she was 2 months old. She was afraid of anyone that sneezed. She would run and hide.
I read up on dog fears and found that desensitizing was the best way for us to go. When I sneeze I just ignore her and over time she had gotten much better. She will look at me, but she does not run away and hide any longer.
I also read try doing activity that the dog loves and cough, several times, when you are having fun. Never cuddle or tell them all is ok, that just reinforces that is ok to be scared. Also do not punish in any way.
Is your dog a rescue? How old was the dog when this started?
Cheryl