I want a promise ring😔

I've been with my bf for 3 YEARS and although he's 18 and I'm nearly 18, we're both quite happy with each other and how we are and if I'm honest, call me crazy but I think this is it. I love him more than anything, don't get me wrong these three years haven't been easy, we've been tested with so many issues including my mental health that he has to deal with, me moving away to go to college and also feeling like we're missing out on "testing the waters" and ensuring that we do in fact want to be with each other. But never the less we're still going strong and if I could I'd marry him tomorrow!

We've both discussed the future and we're both quite happy to keep this going as long as it lasts, however long that may be, but when the time is right we hope to move in together, get married and possibly start a family. Most people when they find that person, get engaged, to say that they will get married, this is who they're committed to and this is who they're going to spend their life with. I want to show this, but I'm too young to get engaged, so I figured a promise ring would be the next best thing. However my boyfriend just doesn't seem to understand this and I can't get it through to him. He just keeps saying that he wants the first ring he gets me to be an engagement ring. Help!?