Listen Sweet heart, you seem pretty young, considering your question, and that being the case, he's probably relatively young, too.
You need to -own- your breasts-- don't be ashamed of them, and don't think there's something wrong with them just because they have stretch marks
. Nearly every woman ever has or will have stretch marks on her breasts, so you have the advantage of getting to be used to them early on. When dealing with a guy, treat your breasts as though they're a special treasure. If you act as though they're to be respected, he'll be more respectful of them as well-- AND if you act like he should feel priveledged to even consider seeing them, he'll feel priveledged when he does.
There's nothing wrong with your breasts, and you need to accept that. You are woman, and you are beautiful.