Please help.

I have two French bulldogs. Mother (6) and son (3). The boy has recently been pooping in his crate at night. Before this, he was fully housetrained.

His last meal is about 5-6pm, let out every half an hour or so, and he’s in his crate from about 12am-6am.

The vet said there was nothing physically wrong. So I’m thinking it’s maybe separation anxiety from his mother, as she sometimes sleeps upstairs at night. My mum refuses to have him upstairs because of him pooping, as she doesn’t want it upstairs on the carpet. He sometimes barks/whines when his mum is separated from him, which is why I’m suspecting separation anxiety.

My mother is now wanting to get rid of him as she’s fed up waking up to him pooping in the crate.

I’m unsure how to go about solving this, as different sites have different approaches. I don’t just want to leave my other dog with him 24/7 as this won’t resolve the issue, and I assume it will only worsen once she dies as he’s seems so reliant on her.

My mum said if he doesn’t improve by the end of the year, he’s gone. So any help would be appreciated