Fear of pooing / with holding poo

[Cheesedog123]

my 4 year old son has been having problems with a fear of pooing for the last year now. We’ve tried various medications through the doctors and have found lactulose to be working the best at the moment. However when it comes to the time of him needing to poo he’s hysterical. He’s suspected to have sensory processing disorder and Autism. Have tried books, videos, all the hints and tips, health visitor etc but seem to have hit a wall at the actual pooing part. No longer constipated and formation of poo is now just right, no leakage or overflow either side. But still only pooing once a week (was previously only every 2 weeks ) which is obviously making the size of the poo huge. Hes still in nappies and has started having embarrassment around the jiggling around his does to hold in the poo around others which only makes him more upset. Any help would be much appreciated, can this problem be fixed or will it be something he will always be fearful from?