I felt the exact same when I first started doing things with my current boyfriend, I know exactly how you're feeling. I was scared that I was too fat and my body was disgusting but when it came down to it, he didn't pay any attention to my body in the slightest. Truth is, I talked it through with my boyfriend and he was very understanding. He was great about it though, to this day he still is. It frustrates him that i've got such a low opinion of myself.
If you are feeling uncomfortable, talk to your boyfriend about it as i'm sure that he will be understanding. Just take things slowly at first and see how things turn out. You don't need to commit to anything just yet, if tomorrow you don't feel up to it, wait and try again some other time. Try not to worry about your body though, he wont be worrying about your body so neither should you. All he will be thinking about is what is going to happen next and I bet he has some insecurites too...