My theory
I believe that the mysterious man (Neal Casady) is the white rabbit from Wonderland.

Why I believe this:
* He received a postcard with the word "broken" on it. One thing that is most certainly broken is Jefferson's hat rather than the curse. Which led me to believe he was another Wonderland character
* Jefferson (Mad Hatter) is a reference to Jefferson Airplane who has a song called "White Rabbit"
* His last name is Casady and referencing Jefferson Airplane's band member Jack Casady
*His appearance is very gentleman like similar to the White Rabbit wearing a waistcoat etc however possibly like the white rabbit he can be manic, timid and sometimes aggressive.

Possibilities:
* He is Henry's father. The promos certainly hinted at that.
* He knows Henry's father and maybe wants to make an honest woman (marriage) out of her because of her extenuating circumstances.

Further thought:
* If he knows Henry's father then who would it be? I believe it might be another Wonderland character. I was thinking that it could have been the March hare who is Henry's father which might then explain why she is very hesitant in talking about him. I wonder then if the March hare in our world is one who might be charming, fun but have around or with but has issues of mental illness (bipolar, manic depressive etc) that either Emma could not handle at 18, felt that was too much of a danger to be a competent father or simply did not want to handle (she wanted something simple and normal to contrast all of the pain, and complicated mess that is the foster care system).

Any additional thoughts?