I had the same problem with our son. Our Doc suggested keeping him upright as much as possible. I think the flat side is getting better. I use 3 different types of jumpers and a walker to keep him up right. When he had less control of his back I propped him up with blankets. I also moved his changing table and put him in his crib so that his flat spot was not down.
Our doctor has stricly said no to walker.
He was mentioning that walker is not good for baby hips.
I am not sure how good or bad walker is but we dont use walker at all.
Thanks