How do I do this?

So I’ve been seeing this guy in secret for about a month and he says it’s a secret because I’m friends with his sister and is worried if something happened it would ruin that friendship and I do believe him. He’s voiced before how he wishes he had met me first. ( we almost dated in high school but a miscommunication ruined it) Now 3 years later we are at the same college and both single. We never really stopped kinda flirting with each other.

But I’m terrified I’m gonna lose him to his ex he recently broke up with. I know they still talk sometimes.

I’m planning on telling him that this has to be an all or nothing relationship. Im confident he wants me but im worried that fear is going to ruin everything again.

I don’t know how to ease his worry and to just date me. I just know I need a clear answer, either we give it a try or that’s the end of it