Poop issues again

My son is three - he had a TERRIBLE diaper rash that happened about 5 months ago, that I had to take him to the hospital for.
Well since then he's had a problem holding his poop in. He'll do whatever it takes to make it so he won't poop. I swear i change like 7 poopy diapers from him a day, it isn't like he'll do a big load each time, it's like a quarter size amount each time.
& he's scared to do it, he says things like "I cant do it" "I can't poop" etc.
& it's reallllllly hindering his potty training. He absolutely will not poop on the potty.

He's on prescription Miralax - which was helping for a while but now he's back to holding his poop.
He went like 3 weeks doing this, and I had to take him to get an enema it was the most traumatic experience for both of us but his stomach was rock hard and full of poop /=
Idk what to do, i've tried telling him its okay to poop, everyone does it, if he holds it in it'll make his tummy hurt. But nothings working ...