Did the vet take a sample of poo to test for bacteria?
I wonder if he has been scavenging when off lead if he is hungry and picked up something to set him off again.
Sometimes it is tempting to start normal food too soon and can set them back, I also try to stick to what is working regard getting them firmed up and not deviate, its boring but worth it, plus 3 small meals is much better, and if he does get sloppy when off lead then lead walk until he is well and truly back to normal, even in garden for toilet breaks its worth keeping lead on if you think he is finding bits to scoff.
Unless you do keep a close eye on him you can go round in circles for ages with recovery, I have found this from plenty of tummy upsets with 3 of them to cope with that eat allsorts when out on walks

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I hope you can get him back on track soon.
June