Awesome job on getting the cats trapped and fixed.
I agree that the SG food was probably a drastic change from what Teddy was used to eating therefore causing the loose stools. If he'll eat yogurt try giving him a wee bit (the plain organic kind) or else some pumpkin (plain canned stuff, not the pie filling one) to see if that helps. Or else you can buy some probiotics from the drug store or health food store or else some slippery elm from there.
Looking forward to the pics.