He could have been scared and trying to guage your reaction? I know I have a lot of ideas about myself and like to try them on; he might have been "practicing" in that way?

I'd be a little worried about "he started talking about the kind of gown he would like," and that sort of thing as that sounds a bit controlling. You're the one wearing the gown and the marriage is generally thought to be mostly for the woman and your wishes, if he asks you to marry him, would count "equally."

Sounds like he has a bit of growing up to do and maybe you have to think and sort out for yourself just how interested you are in marrying him in particular versus marrying/having someone because it's companionable and a nice thing. Why did you break up when you went to grad school? Have you more education than he does, is/was he "jealous." He may have the idea that he's "important" to you because of your illness and that makes him feel better about himself and then, when away from you, gets scared.