toddler afraid to poop in potty

Carrie 💙💙🩷💙
My 2 year old son has been potty trained for a few months, but refuses to poop in the potty.  He says he's scared and so he saves it until I put a pull up on him during his nap or nighttime.  Anyone else dealt with this?  We've tried bribing him and it doesn't work-- ha!