He doesn't get to decide if you're still together or not. He doesn't dictate how the relationship goes. He treats you like rubbish - you decide to leave. That's how it should work. You said
But that's the thing, he says we're still together. I ask if it's over between us and he says "you decide, I'm happy how it is"
What he means is, he is happy having a relationship where you do all the running, all the worrying about him, make all the effort, he gets some gf services at week ends but can otherwise live life as a single man. He doesn't even have to call you or respond to messages. He just lies and says his phone was dead. He is treating you like rubbish, and he is quite happy to continue doing so, as long as you'll put up with it.
If you see sense and leave, he won't care about that either, because he doesn't care enough.
This is not a fault of yours - it is all him. He is a rubbish bf and a pretty dire human being.
When someone wants to be with you, they make it clear. You are left in no doubt. You are worrying about, and fighting for, a man who isn't worth it. Please stop messaging him, and leave him alone. If nothing else, you will see how long it takes him to worry about you or miss you. But no matter what, walk away. You would be much happier single than with this awful man. He will be making your mental health worse, as he doesn't show you respect or care.
Can you build up your support network? Are there any family members you are close to? Any old friends you could get in touch with? Do you have a chance to make friends through education or work or a hobby? You deserve so much better than he is giving, and you will be healthier and happier without him.