Toddler poop troubles

I have a 3 year old son who sometimes has trouble going #2 it's not all the time just once every so many months. I started him on a prebiotic/probiotic gummy that seemed to work wonders instantly for him. He started those a little over a month ago probably. But now he is back to "being scared to poop" he has the urge to go because he will run to the bathroom but absolutely freaks out and will not go because it hurts ( and i will have to wipe his butt because he has poop stains in his underwear and on his bum). So he holds it for days at a time! I've tried pedia lax, and incorporate a regular amount of fiber into his diet. I've also just broke down and put him in a diaper when he starts freaking out to try to help him relax and feel like he can just let it out when and where he is comfortable. It has helped a few times but not every time.What can I do to help him?!? Any suggestions and or advise from experience?! I feel so bad for him when he gets like this 😔