OK, fast forward to the Irish party section when Jack is asking Rose to dance. You repeatedly see his hair go from being brushed back then to being in his face! There is no way this could be just a case of Leo running his hands through his hair!
I am in a folkdance group, and I can testify from experience that unless Jack was using lots of gel (which I doubt, I don't think they had it then, or if they did, that he could afford it), his hair would get blown around a bit.