Let me share something. I come from a background of fortication of the worst kind. I've been with more people than I care to say. My wife has been with one person. Me. Sometimes I cry thinking that I couldn't give her the same gift.
You may say that you plan to get married and that you'll still only have been with eachother, but it doesn't matter. It isn't just a gift to eachother, its a gift to God. Whenever we follow God's law, we worship him with our actions.
Your boyfriend is a 16 year old male. Of course he wants to questions this. Its hard to be a teen and deal with the new pressures of hormones and lusts. But please, for the love of God, be faithful and wait. I promise (and God promises) that you won't regreat it.