I want to get engaged...?

Maybe you could casually bring it up. Not in a way that sounds like you're asking if he wants to get married, but maybe in a way that you're just asking if he's given it any thought. Depending on his answer, you can ask him why he doesn't feel ready. Anything else is really going to depend on what you ask and how he answers, but you'll know how to go from there. Just remember to be calm and understanding.