I agree with that quiet girl. I also encourage you to add in your own things of interest in sex and don't do anything that makes you uncomfortable. good luck
You will not have body shape changes due to a pillow. As long as you are comfortable having the pillow there during sex, it's not an issue.
It's not going to do anything for your butt. If anything, it can help make sex more enjoyable for you and/or him as you've already mentioned. But no, having sex with a pillow under your butt cannot change the shape of your butt. That's going to be a poor, high calorie diet that will cause you to become fatter and your genetics which determine where that fat will store--not sex with a pillow under your butt.