Boy vs. Man

My boyfriend and I recently got back together from a two and a half month break. One of the reasons why we stayed broken up for as long as we did was because I thought I was dating a boy, not a man. He asked me to explain what the difference is and I didn't know how to answer. I do, but I don't know how to word it. Now I'm asking you ladies for help! What's the difference between dating a boy and a man?