Well, girls get erections from kissing too, don't they? They're just not as noticeable. In your experience with men prior to this one, didn't they get erections from kissing?
I don't think it's a sign of anything necessarily any deeper than lust, but it does show there is at least that degree of it. All I think it shows is that he's definitely physically attracted to you. Did you think it might show more than that?