I am 37, and can tell you a thing or two about worrying. I've made more than my fair share of mistakes. The best advice I can give you is: Dont Worry! It will be ok, ...even when you think it won't. Trust me! Most times you get all worried, only to realize everything is fine. If there is a problem, deal with it then. Worrying doesn't help either way.
First things first - Stay away from that guy. Don't call him, don't text him, don't message him, and surely DO NOT have sex with him again if this all blows over and everything becomes fine again. There is something wrong with a 29 year old, ...who says he is 21, ...to sleep with a girl who is 15, ...only to cum in her ....when she asked him not to. Zero contact with this guy.
Secondly- It is unlikely that you're pregnant. Possible, but unlikely. If you are, work through the problems if and when they arise. Stressing out only makes matters worse, because you still have the issues to deal with. I got my girlfriend pregnant when we were both 16. We were both terrified. We ended up telling her parents, and getting an abortion. My family never knew. I don't regret it, but if I knew then what I know now, I would have kept it. My parents might have been mad, but they would have got over it. I know now that things would have worked out fine. There would be challenges, but either way is totally doable.
Thirdly- The odds of you getting an STD are slim. I'd advise you to get checked out, but make this the least of your worries. I'm none to proud to have bare backed a couple of really skanky girls in my day. I did the same thing, freaked out at every little pimple. Do not search the Internet about STDs in your state of mind. It will only fill you with fear, because you're hearing the worst case senecio most times. If you're ever worried about STD's, go to the doctor, and avoid the Internet. Doctors will tell you the same. If you got something really serious, you will know it, or you will find out after the test....which will most likely end up negative anyway. If I haven't got anything with all the crazy things I've done, ...I doubt you have. Deal with it when (IF) it happens. You're most likely worrying for nothing here.
LASTLY!!! - This guy is trouble. He should get in some serious trouble for what he has done with you sexually. Only you can make the decision on weather or not that happens. Only you know the real story here. He may be the type of dude to scavenge the Internet looking for young girls to lie to and have sex with. Maybe next time he will trick a 13 year old. I would put most of your thought into dealing with this situation. Whatever you decide, stay away from him. I wouldn't get him in trouble if you lied to him, and he thought you were 18. However, if he knew you were 15....that's bad.
No matter what the outcome of any of these senerios, just remember: Everything will be OK! Don't worry! It won't be the last time life throws you a curve ball. Instead of worrying, apply that energy into serious problem solving ability! Don't be afraid to be yourself! Don't be afraid to tell the truth! Don't be afraid to speak your mind! It's your life, and everything will be OK no matter what happens in any of these situations! Good Luck!!!