Baby flipping himself naptime

My 6mo has discovered the art of flipping himself any time I lay him on his back! This is awesome except now when I lay him in the crib on his back, he immediately flips on his tummy and gets frustrated. He’s very tired and needs to sleep so idk what to do?? I know he can sleep on his stomach if he can roll over but he’s just frustrated and I’m worried about him spitting up since he just ate recently

Update: he ended up spitting up. Ugh this is exhausting and it finally took 2 hours of trying for him to finally fall asleep