number one: you are not an asshole
i would like to point out that it's true you didn't set boundaries.. however, you might not have been at a point in your life where boundaries were something you regularily set..... now they are... you've changed that's all.
So don't be too hard on yourself for that, but now you are most likely dealing with the consequences of not drawing those lines in the sand.
i would like to take a small detour here, as to why i think you might have let this countinue for such a period of time:
i think, from what little i know and understand about you... is that you like to make people feel comfortable, you like to make them feel happy, in essence, although it bothered you the way he was flirting/acting, you sacrificed your own comfort to make someone else happy, but you're stronger now in your own skin and you've decided not to put up with sacrificing your own happiness for anyone anymore. i would like to point out that this is a big growth step and something to be proud of.
my take: don't call him back if you don't want to. you don't owe this guy anything. if i didn't want to call someone back.. i just wouldn't. I'm fairly sure that this person would get the clue that you didn't feel like explaining anything. You don't have to explain anything to him, he probably already has a sense of what happened, if he's slightly intelligent, he can put two and two together over what the impeteous was over your change towards him.
If you feel you need the closure on the situation. Call him back and just explain quickly and in as little words as possible that his behavior had made you uncomfortable for some time and you weren't sure how to address it and now, you feel that it's inappropriate to be friends due to the fact you're obviously grown into two different directions with your lives.
Have i ever had to do this?? yes. It was with one of my good friends a few years back... she had grown overly-dependent on me and bascially smothering me (we all know the type) I just frankly said, well, you know, i think we're two different people now. It was a longer process than just that talk... however, she made a new best friend and the situation resolved itself most uneventfully, as i imagine it will for you too.
sweetpea