[canana]

I highly suggest taking him out before bed, especially if he has this overnight pooping issue.

Have you noticed a change in the times he eats? Or any changes to activities (i.e. is he more active at night after his last walk)? Sometimes a lot of activity / playtime can trigger potty. Or if he's eating later in the day, maybe that's why his poop times have changes.

Under normal circumstances, the easiest way to know their poop times is by controlling their feeding schedule. But in your case, it's kind of hard to say.. and it's difficult to correct because he needs to be free-fed due to diagnoses.