While I have no idea why this is found on this forum, I can't help but reply. Honestly, I don't think that the fact that he's not that interested in you makes him a douchebag. I think his reply is pretty honest. No, it's not what you want to hear but he's being thankful and honest that he can't hang out. I'm not sure what other type of reply you'd expect. He can't be too honest or it comes off as rude and he can't be too nice and tell you he can hang out because that would lead you on. From everything you said, he doesn't sound rude at all. He just sounds like he's not trying to string you along or hurt your feelings.
It just sounds like you're both on different levels of interest for each other and what you are looking for. I wouldn't take that too personally. I think it's pretty obvious he's not interested in what you are looking to pursue, which seems like it's a more monogamous relationship, so just cut your losses and move on.