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.
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.
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