Yeah, I'd not let him be anything more than a casual friend. Plus, he's only 18, and not a very mature 18 - he likely doesn't even know himself what he wants from the sound of it. An engagement is not something to undertake - and then break - lightly. He needs to get his own head on straight first, and you don;t need to be anyone's "rebound" relationship.

You deserve better, and I am glad you know that. We are proud of you, kiddo.