First of all, not to lecture, but are you sure that you are ready for a baby at seventeen? Have you finished school? How will you support the baby financially? Do you both have good paying jobs? What will the baby do while you are working or at school? Do you two live together or do you still live with your parents? Do your parents know about this plan? What happens if the two of you don't end up staying together?
When it comes to the burning, it may be possible that the burn is actually from friction during sex, and you just don't notice it until he finishes because you are too wrapped up in the act while its going on. If this is the case, try using a lubricant to help ease the friction; it will burn a lot less! Also, there are some cases where girls are slightly allergic to a guy's cum, and she can get rashes if it touches the skin.
You should go to your obgyn and talk to him/her about the problem. They are knowledgeable in the subject and would know how to help you better.