Promise Rings vs Engagement Rings

My boyfriend is currently deployed and told me he is gonna propose to me when he gets home (next month). I love him but it is way to early for an engagement and most of our relationship, he has been deployed. I'd rather him, give me a promise ring when he gets back and wait on the engagement... How do I bring that up with out hurting his feelings or sounding like a gold digger for asking for another ring?