I'm quite sure that many here would disagree with me but I've had Thor for 3 and 1/2 years now and have dealt with loose stools/diarrhea from day one. Thor demonstrates his stress, his angst if you will, with a bout of the most vile diarrhea...pools of it, even when he was in his crate. Some GSD's tend to have 'touchy' tummies. Thor spent his entire puppy life until age 11 months tied to a tree, with no socialization, no training, bad food, basically a stressful life for a puppy.
I discovered that there were several things that worked well in solving the problem.
#1 He gets only boiled rice, sometimes for two days, which seems to tighten his bowls. Then I add some dry food to the rice with some warm water. No meat of any kind until he's over it.
#2 If it still doesn't clear up, (which is rare anymore) he gets one tablet of metronidazole. I have a permanent supply, thanks to the vet, who has checked Thor thoroughly and has found no physical reason for his problem...I'd advise you to see your vet and verify that your foster has no underlying physical reason for this. The combination of rice and the tablet has never failed me.
#3 I try and determine what is causing his stress. New people in the house? Relationships with other dogs in the pack? Major thunder storms? It could be almost anything that is not normal in his routine. Using the first two tips, and searching for and correcting (if possible) what might be upsetting him usually sets him right. Other than the early days when he first came to me, his diarrhea seldom lasts more than a day, two at the most.
I do hope you find what works for you and your dog. I'm sure you'll get plenty of tips here. Find out which one works for your foster. There is nothing more appalling than waking first thing in the morning to a lake of diarrhea that needs cleaning and a dog who looks so ashamed of his behavior. I've NEVER (very important) scolded Thor as I know he can't help it, but HE knows he shouldn't be doing his business in the house. I reassure him he's still my good boy and we just clean up the mess with no fuss....after we've taken him out, of course. *grins*
Good luck!!