Hi from potty training hell

Potty trained my 2.5 year old son two weekends ago, nakey weekend, using the oh crap method. He took to it SO well. Zero accidents. Has been great. Scared to poop, though. Will now hold his poop and it’s causing him to have pee accidents. It’s TERRIBLE. Help!! He’s in a pull up for nap and night but won’t even poop in that now. Have tried miralex and magnesium. Trying to make it impossible for him to hold but it’s not working. Traumatic for all. Help!!