I think one thing that might be helpful to keep in mind is that people are, in general, attracted to someone if they're in a relationship with them. I don't want to downplay your own feelings about your body at all, but there's no specific weight or body type needed to be attractive to someone; you might feel insecure but those feelings don't necessarily match the feelings anyone else has about you. I shared some body-positive links in a thread over here:
http://scarleteen.com/bb/viewtopic.php?f=21&t=2321 if you think any of that would be helpful.
Having said that, though; it's ok if you're nervous. Many people are nervous to some extent the first time they're sexual with a partner, or naked around them, and that's normal. If you're feeling
really anxious about this, though, instead of a sorta-anxious-but-mostly-excited sort of feeling, I think that's a sign that you might not be ready yet. We have an article about assessing readiness for sex that I think would be good to read:
Ready or Not? The Scarleteen Sex Readiness Checklist
I have a few other thoughts. One is that sex encompasses a LOT of activities. Sometimes people will treat only intercourse as "real" sex, and if that's specifically what you're meaning when you are talking about having sex with your boyfriend, I think it might make sense to step back a bit and consider the really wide spectrum of sexual activities you two can engage in (including ones in which you're not naked, if that's really worrisome for you right now), some of which might feel a little safer or less nervousness-inducing than intercourse. Some articles to look at when thinking about this:
What's Sex?
Yes, No, Maybe So: A Sexual Inventory Stocklist
Yield for Pleasure
The other thought is this: it sounds like it's really important to you and your comfort for your partner to be
very aware and respectful of your boundaries. Do you feel like he's at all pushy when you talk about having sex, or about your nervous feelings? Have you set boundaries around sexual activity in the past, and has he respected them without trying to cross the lines you've set? If he is pushing against boundaries at all, or just ignoring ones you try to set, then that's something that could certainly be contributing to your nervousness.
And if he is already being really great and respectful, maybe having a really explicit conversation when you talk about how you appreciate that and maybe try to talk really clearly about what boundaries you both have, so you can both be on the same page and feel a bit more secure knowing you're in accord on this, will help make the situation a little less scary.
I've given you a lot to look through and think about here, but hopefully at least some of it will be helpful.