Iguana Diarrhea?

I have a 5 year old male Green Iguana that has just recently started having severe diarrhea (for the past two months or so. Yeah, he's had it that long). He's been tested for parasites twice, and the results have been negative twice. I've read everything there is to read about it. The vet said he didn't appear sick. No extensive fluid loss, no mood swings, no yellow/black coloring, and no behavioral changes at all. Nothing. Nothing about his environment has changed to stress him out.
I've had him since January, and before that his diet was really bad. The guy fed him collards and grapes every day for the year he had him. I'm surprised he was in okay health when he came to me. Could this just be the effects of the bad diet?
I've tried tweaking his diet. Everything from more/less fruit to canned pumpkin. It hasn't changed. The only thing I have not tried is animal protein. Everything I've read says they shouldn't have any of it. He won't touch mealworms, insects, pinkies or mice (alive or frozen).

Has anyone ever heard of giving them those viena sausages to balance out his diet? Maybe he's just not getting enough vegetable protein? I have no idea. I just want him to be okay. He's my pride and joy.