Totally the right call to kick the man-child out and get out of a relationship that doesn't work for you. The dude sounds like a loser. At best, you are at different places in your life where you have a different view of what adulting looks like. You can't drag someone kicking and screaming into your version of what life should look like, and frankly you shouldn't have to do that with someone who truly cares about you. To make myself perfectly clear, you are making the right decision.
That being said, what did you realistically expect to happen when you are the only SO at a reunion of his pothead dress-up pals you admittedly don't care for? You gave him the "just one more convention before you get your shit together" ultimatum, he decided to treat it like it was like any other convention, and you're upset that he did? Let's assume he isn't a total jerk and told his friends that because he loves you and wants a future with you, he won't be attending as many conventions or making the costumes for free anymore, because he has to make some changes to build a life with you. Assuming his friends are like any other group of guys, when they're away from their SO's they will devolve into overgrown teenagers making crude jokes and indulging in things that would get them in trouble at home, and now one of them is being p-whipped into not joining in the fun any more. I could see where while in a room full of his 'bros before hos' drunk / high buddies, he would be a complete dick to you on the phone just so his buddies (who obviously didn't want the Debbie Downer who is trying to keep their fun buddy from hanging out with them there to ruin the weekend) would stop giving him shit. In my younger days, I personally made that mistake once or twice. It's NOT acceptable behavior, but I can understand how it happened. The guy was in a no-win situation and was going to piss off someone. He just made the wrong choice if his ultimate goal is a relationship with you.
I also don't have an issue with taking a sick day or two for personal reasons. It's not like he's a surgeon and someone is going to die because he wasn't at work that day. He alters clothes, for pete's sake. As long as he has the rent money on time and you have separate finances, you don't really have a dog in that fight.