I mean I don’t know. It hard to figure out when were not there and have to guess.
It could be that he wants what he can’t has and when he has he losses interest.He could have interest but interpreted you not as interested and is hot and cold based on that (I’ve done that where I’ve lost self-esteem and ignored a girl but tried again when I thought I saw signs). It could be a lot of things. We won’t know.
I’m assuming you’re young by your story.
Group plans is way too casual. And I get you want to make it light and casual as possible but that can be interpreted very easily as “friends” type stuff (if not done right with flirting etc). You’re not going to like this answer but you’re going to have to be vulnerable and actually flirt and be possibly rejected (if he likes you just as a friend or is playing hard to get). You don’t have to be weird about it or directly ask him out. You can say something like this [someone in the conversation] touch his arm/shoulder whatever and say “Gee, when are you ever going to ask me out?! You’re as hot and cold as I am with clothes!” [say in a flirtatious, casual/lighthearted, joking way that teases him but also confronts him in a non-emasculating demeaning way]. It shows confidence on your part which is great. And plus you’re not asking him out (you are) but you’re saying indirectly “hey bucko if you don’t don’t ask me out, I’m going to walk [if it doesn’t work be prepared to walk. do not hang around this guy hoping he comes to you].”
Make whatever you say so it can’t be misinterpreted (like your group friend hangout thing). And make it so he has to make a choice (yes or no). But you have to make it casual and lighthearted. Do not make it a big deal and pull him to the side or anything like that. You got to be open and welcoming as much as possible. And if he doesn’t he’s a) not interested or b) not ready (to which you don’t want to date him anyway). If he’s hot and cold that stops it because it forces him to make a choice in actually dating you and getting behind the games (which is what you want). If it doesn’t work out there’s other guys.