Thanks, everyone.
Things seem somewhat improved this morning. A friend from here called last night also suggesting pumpkin and metimucil (sp?), which we will try today, too.
As for the Metronidazole, that is also what I thought he'd need and the vet said she'd need to come do bloodwork before prescribing, which is part of why she came out last Thursday, did the exam and bloodwork and told me he doesn't need Metronidazole.
He had never been on antibiotics before I gave him the SMZ's yesterday. His history is that at 16hrs old and 6wks old, he had hyper immune plasma transfusions. Not because his IgG was insufficient, but because last fall, I got to sick / dying colts in from another farm. Now we do this for all our babies to be sure they stay healthy (it's actually not uncommon for high dollar TB foals to get this as a precaution and after what happened with the colts I bought last year -- not taking chances with our babies). The plasma has "protection" for just about anything a horse could get. Trooper got about 1/4 bag at 6wks old because he had an anyphelactic (sp?) reaction (as did one of our fillies at the second transfusion) but it still was enough to boost him like we wanted.
He had loose stool 2 days prior to the 6wk transfusion, but during the reaction, he had liquid diarrhea which the vet said was nerves (probably so). However, this was the start of his weekend of projectile diarrhea (last weekend). Started BioSponge on Monday (20cc 3x day) for things to firm up significantly but with continued BioSponge, returned to liquid on Thursday prompting our call to the vets requesting Metronidazole to be told they needed to examine him and draw blood, which happened Thursday afternoon.
He did have a mild "foal heat" diarrhea which of course wasn't connected to his dam's heat as he was't nursing from her, but nothing that required treatment. His poop used to stink more than I think it maybe should (going back to when he was a week or two old) but I attibuted that to him being on formula vs. mare's milk.
If the practice has any new revalations this morning, I'll let you all know. For now, though, he had somewhat firmer stools last night and didn't have a gross butt this morning (phew). Planning to continue all of the changes / additions we've done and add in the pumpkin and metamucil this afternoon (H will pick it up on his way home, I'm planning to be here all day).
The brightside (other than how much we love Trooper) is that when we're all done with raising him, I'm going to know way more than I ever did before about raising an "orphan" (mom rejected, but w/ circumstances) foal and about diarrhea in horses. We've never dealt with either in all the years of having horses.