I wish there was some way to interpret his behaviour. It could be what you are afraid of but it could be that he's in NRE and inconsiderate. Or it could be something else. I think you can have some idea intuitively, since you know him; but then again, poly situation where he has another partner is new to you, so if you don't feel certain, you shouldn't make any rash decisions based on that feeling. However, there are limits to how much people here can help you to know what's going on. If he isn't telling you, that's a problem in itself.
Maybe you could shift your focus, though? What if, instead of trying to figure out what he wants and means and is thinking and not telling you, you focus on yourself. How do you feel? Where are your boundaries? What kind of treatment are you willing to accept? How inconsiderate is he towards you? If he has chosen to not talk to you (whatever the reason), what are you going to do with that?
You can't make him communicate, he has to want to. You should communicate to him that you take it very seriously when he cancels plans with you to spend time with gf. But if he still does it after you've done that, you can't make him stop, you can only decide how you react to that.
I think that if time and my attempts to communication didn't cause my partner to change their hurtful behaviour, and if he refused to communicate, I would take a step back emotionally. I wouldn't throw in the towel right away. But I would think about the issue in terms of if his behaviour is something I can live with or not. If not, I would choose a time, in my head, and say to myself "if things haven't changed in x amount of time I can no longer stay in this relationship".
His behaviour is, ultimately, in his control, you can only control yourself. You should decide where your boundaries are, i.e. what kind of treatment you will accept, and react accordingly.