Dirty dog!

Would it be possible to leave a door open for him at night, so that he can go out to poo? Or put him to sleep in an outside shed etc?
It's not cruel, our collie used to sleep outside in her last years.

He's fed twice a day,two thirds morning then rest teatime. I will try giving him his second feed earlier. I can't think of anything that's upsetting him,he's his usual self apart from this. Thank you for your replies,I thought I may be missing something obvious. Let's hope it's a phase!

If he's fed once a day, change it to mornings instead of dinner time.
Also stop any treats unless teeth cleaning stick, (given before 12 noon) and stop the scraps from your left overs.
If you normally feed him twice, drastically reduce the evening feed to a tiny amount.
It seems to me that he's being fed a lot, late in the day and can't hold it all night now he's a bit older.
I would say the problem is around food anyway, either too much or given too late.
Hope this works for you.

Have there been any changes in the family, or at home? I'm wondering if he is upset about something.

As the vet says he's fine, there obviously isn't any physical problem so I think something is worrying the dog.