Kittin Diarrhea

I found a beautiful stray kittin about 3 weeks ago. He has had non stop diarrhea from day one. I took him to the vet's and my veternarian said " The probalem is definately worms"  so I gave him the worm meds as directed and i was told that after the last dose I would see much improvement... and I definatelly have not. I have tried alot of home remidies and none of them have worked so far(pepto, rice milk, wet food , dry food) and nothing seems to make a difference. The vet confirmed that the kittin was 9 weeks old when I found him so I know he's old enough to have solid food... he started off eating purina kittin chow and then I switched because someone told me that cats can't have soy and almost every cat food has soy in it asside of a select few brands so now he is eating the good life recipe but I am absolutely stuck I have had cats all of my life and i have never had a probalem like this. He plays and seems happy, but comming out of the litter box his lil but is still dripping his poo is so bad. I feel so bad for the little baby and i just don't know what i can do to help him.