Puppy barking all night

Hello

I have an 18 week Jack Russell who on the whole is gorgeous and intelligent and a joy to be around. However I have this recurring night time problem that is driving me insane.

its defecating in his bed and then barking all night. When I first got him he slept in a crate in my room for the first week, I then moved the crate downstairs with very little issue. A few barks for the first few nights but nothing that lasted longer than an hour and after a week or so he was sleeping. I was getting up at about 3am so he could go to the toilet outside and then he was back in his crate no problem. I felt very lucky. I should say as well that apart from this his housetraining has been great and we have very few accidents now.
However things have taken a very difficult turn in the last few weeks. He’s started pooing in his bed. At first it was small solid poos that were outside the crate (he was sticking his little bum through the bars) and I was coming downstairs to find them in the morning but now he’s barking and doing larger poos in the bed itself. It obviously distresses him because he gets very agitated, even after the mess is cleaned up he will continue to bark for hours.
Most nights he sleeps through, I would say this problem has happened 2/3 times a week for the last 3 weeks. This is how it goes- he starts to bark so I go downstairs to take him out. We stand outside for about 15 minutes (he’s usually barking and excited, I try not to give him any attention when he’s like this at night). Nothing happens, no poos, maybe a wee. Eventually I put him to bed and the barking continues, after another while I come downstairs again and he’s defecated in his bed. I clean up, but him back in his crate, the barking continues. Usually until about 4.30am.
I’ve tried everything.I’ve moved his last feeding time forward, and try not to give him any food after 6, I take him for a walk in the evening to get him to poo before bed (he usually does but not always). He has regular exercise and we do training and brain games in the day so he’s tired by the night time. I come downstairs regularly to take him outside in the night.
Nothing is working and I’m losing my mind! Any advice you can give me would be very much appreciated.