My son had this problem with pooing we had a regular 15 mins after tea we would put Vaseline on his bum hole and then he would sit with a stool he hated his feet dangling and watch something on the iPad or phone I would always sit with him if he wanted me too and that worked for us we also invested in a family toilet seat as he felt safer on a smaller toilet seat. Lots of praise and encouragement we used to have private poo parties when he did it meaning cake! Hope this helps its hard work but keep it up