An idiom for a very sensitive person

Is there an idiom for describing someone who is oversensitive (emotionally, not physically)?

I vaguely remember something along the lines of "he is all exposed nerves".

to be a bundle of nerves
to be wound too tight [sic]
high-strung
too tense

Then there are loads of these types:

Nervous as a long-tailed cat in a room full of rocking chairs
Nervous as a porcupine in a balloon factory

etc.

You might say that you had to "walk on eggshells" around him, implying that the act of interacting with the person required delicacy and was difficult to accomplish without causing show sort of blow up.

The closest I can come your suggested idiom is "you've hit a raw nerve", which is sometimes said when something has been said to a person who is sensitive about a particular topic. This does not necessarily carry the implication of sensitivity in general, though.

I remember getting together with some former coworkers years ago; we were catching up on old news. My friend asked:

What about Sam? Is he still a quivering mass of insecurity?

This isn't a common idiom, but it's wonderfully colorful language nonetheless. There aren't too many utterances that I remember so vividly, word-for-word, even more than 20 years after hearing them. But sometimes a fresh metaphor beats a well-worn idiom.