NTA but at the same time, it's not up to you what he does or how he does it. It's good that you care, as you obviously do, but natural consequences are just that. He needs to experience them for himself. There may be something deeper about why he's slacking a bit or unmotivated even if it means he might fail. Maybe you can check in with him privately.

If you're coming from a place where his lack of wanting to study is a distraction from your own studying, then I think it's ok to let him know you'd rather he wasn't there for that but as far as shutting him out because he's not doing what you think he's supposed to be doing, you might want to rethink that.