all most 4-year-old pooing in nighttime nappy

Hi everybody

I'm looking after my sister's boy for a few weeks while she takes a break, she has never used disposable nappies so has given me a bunch of cloth nappies as the lad is still not dry at night. The issue I'm having with him is when I go to his bedroom in the morning to change his nappy (he would have been up before me playing sitting etc for some time) not only is the nappy wet but there was quite a lot of squashed poo in it as well sorry TMI, I thought it was a once off, but this second morning it was the same.

When I ask him about it, he shrugs it off saying his mum doesn't mind it.

It's her only child so wondering if she is just babying him too much, not sure as I haven't spoken to her about it as yet. I don't want to tell him off but feel I need to do something. I've not noticed him doing poo's during the day so wondering also he could be saving it for his overnight nappy.

Any suggestions.

Leigh