I am not a teacher.
"Chest hair" is uncountable and takes the singular. "His chest hair is showing."
If you mean individual hairs and not the hair considered as a whole then it is countable and takes the plural. "His chest hairs are showing."
To say that someone's chest hair is showing would imply that it shouldn't normally be visible but it is. If someone were bare-chested then their chest hair would simply be visible/on show/on display/ and so on.
Student or Learner