How do I stop my son from holding in his poop?!?

Rebecca • 27, TTC my second baby. I love my husband for giving me baby #1 who is the most amazing gift in the world!!

He was put on Movicol at 10 months old for constipation issues and finally came off it at 2 years of age. He’s 27 months old and recently had a bad bout of constipation because my family kept feeding him toast (white bread) atleast it only started when he started wanting and eating toast constantly!! Obviously it hurt when he finally went because it was hard and now he’s refusing to poop!! He will stand there and hold it in and I don’t know what to do to make him go. He eats weetabix for breakfast, I’ve moved him onto brown bread, he eats fruits and veg, absolutely loads of water, I’ve started giving him prune juice in a morning and I just don’t know what else to do! I don’t know how to stop him from holding it in. I’m in the UK and the doctors just said a high fibre diet which he already has. I’ve tried reassuring him about going but because he’s holding it in I’m scared it’s gonna be painful for him when he does go and it’s like the cycle starts all over again 😢