Baby sleep help!!!

My son just turned three months old today. He sleeps in a crib no blankets or anything in the crib or bumpers totally a safe sleep space. He keeps rolling into his belly while he sleeps, he doesn’t have great head control which totally freaks me out, I reposition him every time I see him on his belly and within two minutes he goes right back. I’m totally freaked out I don’t know what else to do, this is literally the only way he will sleep, it’s like he’s on his belly but hasn’t figured out how to move his arm so his top half is still like on his side if that makes sense, I’m not sure what to do or how to make it so he stays on his back.