With diet and exercise, his shoulders will probably grow stronger and his loin might fill a little, but not much will change because he's already built the way he is. If he doesn't have promising width now, he won't do much now at 12 weeks. I've had depth develop (Have decent depth to begin with) over time, but not width. You either got it, or you don't. Faults like pinched hindquarters won't change over time. The biggest advice I have to offer is start from the back and then move forward with your breeding program. Eventually it will all come together. It will be hard because, yes, he is a rare breed, so you are going to have to select some wow factor purchases. Lilacs are a really beautiful breed!