He's been to the vet and fecal was negitive...BUT STILL HAS THE RUNS, We are on day 4!!

I've posted on a few other threads about my situation, I have flagyl...i'm scared to give because of the side effects.
FYI PUP EAT, DRINKS AND VERY PEPPY.
He is teething, not sure if this is related,

he had 3 doses of Kaopectate, once last night and twice today. It's still VERY soft... Since he shows the signs of all is well, if there are no improvements in the poo tomorrow I'll go ahead and start him on the flagy.
From what i read get him on a probotic with it. I only have enough doses for 4 days, what i also read it that, it may take 10-14 days in doses???
Any suggestions, he's been on we haven't changed Chicken Soup food, good stuff? I gave him a few tbs. of I/D vet said, he doesn't like it. Why change the food when they are eating it????