With brazilian butt lift (which is what your Dr has suggested) you have to be really overweight i.e. have a lot to work with in terms of fat, to really make an impact. Plus then the fat re-absorbs itself after the surgery, and I've also read that the freshly stored fat has lowest survival rate, i.e. unless it's been in your body longer, you're likely to lose it the quickest.
have you thought about butt augmentation maybe?