4 year old starting to poo withhold - help!

[twolittleboysonetiredmum]

Our recently turned 4 year old (July - starting school in September - probably relevant) who potty trained about a year and a half ago with no poo issue etc has started holding poos in for 3 to 4 days at a time then having an accident. He's just pooed himself in his sleep and woken up crying :( im totally perplexed by it all. His older brother used to withhold but that was from potty training. I've tried to bribe him to have a poo over the last month or so and treats etc hold no appeal. Any suggestions how we can help him? He has a few days of just feeling awful until he poos too she obviously finds having accidents embarrassing and upsetting. I'm really worried he'll have an accident at school or similar. And mainly that it's a reflection of how worried he is about school or something similar if he's witholding. Any advice or similar experience?

Our quite normal, my dd who's now 5 is going through poo withholding for the second time
Doctors will usually prescribe movicol which is a stool softener, a lot is fear of the poo hurting when it comes out.
I try not to make a huge issue of it, offer to sit with her in the toilet, I've also used suppositories available from chemist or GP, the child ones, it's mostly psychological but a visit to the GP to make sure all is ok is worth doing.

Why do they do it? He's never done anything like it before. He's quite highly strung (gift from me I suspect) and is proper stressed about it. I'm a bit worried about giving him movicol etc as maybe it'll mean he's not solved the problem? If that makes sense? And then we'd be stuck In a cycle with him having to have movicol etc

Google a book called Poo goes to Poo Land (think you can download it for free).

Why do some kids have night terrors, why do some have fear of the dark.
I'd not be worried about giving him movicol, if he is withholding then the poo becomes hard and can be painful to pass, therefore hurting when he eventually goes, giving him more fear and reason to withhold.

I also line the toilet bowl as dd was scared of the splash

Also it's rare to be stuck using movicol long term, unless there are medical issues for needing it, I used it with dd for 6 months as that's how long her poo phase lasted.