promise ring?

My boyfriend and I have been dating for almost a year and a half now and we always talk about us getting married but we're a bit too young to get engaged and I have recently come to love the idea of a promise ring. The thing is that i don't know how to bring it up to him and don't what me mentioning it to him make it seem like I'm pressuring him to get me one or something. I kinda just want it to be his idea but I don't think that he really know much about them. Any ideas or suggestions could help