I guess I should refrase that... These people grow it and it is relitively the same everytime. Also it is not a set product that we use for worming. He is on a regular schedule but we use alternate products through the year as you are supposed to.
The first things I would have checked are what had changed in his routine and feed that might cause it. Since you've said he's had the same routine and feed, and everything appears normal, those aren't factors. As long as he is not getting dehydrated or losing weight or mass, I would agree with your vet. You could try probiotics which may help, but usually it works itself out.