Yep, desireless, you have made good efforts (ha, I sound like a school teacher now), but you were always pretty vague. Obviously not from your point of view, because you knew that you wanted him to be there.
When you invited him for dinner, I wouldn't have showed up either (in the US) because you indicated that you were there with a group of friends and he was going to be the 'intruder'. To me, this looks like one of those 'polite invitations' that are generally not expected to follow up on.
The invitation for coffee had 'some time' in it, which again is pretty vague. You could have picked up on his mentioning a specific place and suggested to go there together (this is what I would have done, but of course I know very well that not all men appreciate such initiative from ladies!).
Forwarding a dinner invitation from a colleague, again you had good intentions, but doesn't seem either you want him to be there, but are being polite.
I write all this from a German point of view that has been exposed to Americans and this is what I would have felt if I were this young man.
You definitely have given him a few opportunities to do stuff, but you were never very specific (that's what Germans want, we want to feel loved for ourselves, and not because we make the dinner table look well occupied ;-)).
However, he also should have realised that you are fairly persistent (as in you care about him, not that you are being annoying). Try one last time (being specific!!) and if he does not take the bait, it's time to give up on him, I'd say.
(I had to use English for that, German is not my language of choice when it comes to advice of that kind ;-))