Hey, serious crayons!
You know what, after that HD video on Youtube of Ennis in Jack's closet, i do think he says "Oh, Jack" when / just before he touches the pocket of Jack's shirt.
I also see his mouth moving and hushing something, what you say is an "i love you", but it feels to be sooo quick and unnatural that i chose to think that he is not saying it

. But that's just my choice.

This thread was extremely illuminating, by the way!