Which is best

Cassie โ€ข ๐ŸŽ‰๐ŸŽƒ๐Ÿ๐Ÿ‚ blessed with a beautiful 3 year old boy a 2 year girl and a 9 week girl๐Ÿ‘จโ€๐Ÿ‘ฉโ€๐Ÿ‘งโ€๐Ÿ‘ฆ

I've always been told to lay baby on his back, but I'm afraid that he would get a flat head for constantly laying on one area, I have a friend whose baby's head is litterally flat from constant laying and it doesn't look like it's gonna change, the way I see it is sometimes I lay him on his side during his sleep also It helps me keep calm if he is sicky, he lays on his tummy when he's sleeping on me and I was wondering if I put him on his play mat belly down would that be ok for him to sleep. Also is it ok the way I lay him to sleep or does it have to be his back :/ Hes 4 weeks and 2 days today, so any glopeople with your answers would be great help :) xx

His play mat but sleeping on his back x