DS struggling with being changed

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My DS is seven and has ASD. He suffers with constipation and loose stools. He isn’t toilet trained and is still in pull ups. He is very resistant to being changed at the moment and will become distressed and angry when we try. Any ideas what could help?

Could DS be sore? If that’s a possibility changing the brand of pull up, wipes, cream etc. could help. If DS at a point where he is able to help that can sometimes reduce distress.