I dont really have any advice from experience, but anyway, just wondered if he isnt wedged up enough ? could you maybe try and roll an extra blanket/towell to help prop him up.
I am trying to get my son to sleep on his side as he's developing a bit of flat head so was advised to try and get him propped up and onto his side. Well he hates it, he screams when on his side and i dont really have the patience at the moment to sit with him to settle him. He's a back sleeper and i have him on a sleep positioner which helps flathead so i'm not really going to push it. I used the positioner to prop him on his side and then put 2 thick blankets rolled up together on the other side of him and it kept him in place.
I heard that at 6 months babies tend to change positions in their sleep and some tend to start rolling on their tummies to sleep.
I am not getting much sleep at the moment so i can understand why you would want any kind of advice.
Louise and Luke - 15 weeks