Side sleeper?!! Help!!!

Yolanda 👽 • 🇦🇺 | IG: what_the_playtime |💍| 💙👨‍👩‍👦‍👦💙 7/8/18 + 24/2/20

LO is 5 1/2 months and can roll back and forth during tummy time. However he sometimes needs help rolling back onto his back during these times and now sleeps on his side. I put him to bed on his back but he naturally goes to this position after just minutes of being put down!

I'm freaking out. He's never done this and I JUST transitioned him to his cot. HOW DO I GET HIM TO STAY ON HIS BACK!! 😭

Picture of Liam for reference: