Most Helpful Guy
know this - once you send a picture like this, he will show his friends, or, worse yet, he will put the pics on the internet and will be there forever. Tell him there are plenty of naked women on the internet for him to look at. Don't cheapen yourself.
Most Helpful Girl
Tell him "No that makes me uncomfortable and I'm not going to do it". If he complains about it after that or asks you again you know he doesn't care about you and your wishes regarding your own body and you'll need to dump him.
It's your body. You are the ONLY one who gets to make decisions about how you share it.
Also please don't send nudes until you're over 18. You don't want to get in trouble for distributing child porn.