Poor Soul. Poor you.
A few things, based on my experience with Minnie (who has IBD):
1. I'm not a huge fan of fasting, because it seems to make Minnie worse, not better. However, vets often advise it to give the bowel a rest. And, you're right--the less he consumes, the less likely it is that he'll need to poop.
2. The other thing that can bring on a bowel movement is exercise, so you might not want to walk him or let him motor around the yard.
3. Attacks don't necessarily happen immediately. Even at her sickest, Minnie could go a week or so between bouts of HGE--the meds and restricted/special diet would hold off the symptoms. Eventually the cumulative insults to her digestive tract would result in a bout. Once she was healthier, her system could handle the stress for much longer before it cratered.
The delays make it extremely hard to isolate the problem. It could be that the venison (or potato) in his regular food is an issue, but it just took a while for the reaction to become apparent.
I hope that Soul has a good night, so that you can deal with your regular vet tomorrow. One of the things you might discuss is keeping an inventory of flagyl. I don't keep it, but I do consider tylocine and misoprostal (stomach protection) stock items. Sigh.