Mine ia the same for one week i put her to sleep on the other side and start getting better was a struggle in the beginning but i managed with a blanket folded in 4 tucked under her right side and that way she had to turn left after 2 nights she started sleeping on her own on left side. Other than that tummy time mainly.
My oldest was the same. Once she started sitting up all the time and also no longer sleeping on her back it sorted itself. She's 5 now and you'd never tell she had a flat spot x
Try sandwiching his head between two things. I know it sounds harsh but might work only do it when he’s napping maybe so you can keep an eye on it. I know you can get pillows for this sort of thing.