Help, My 4 Month Old Pup Started Having Diarrhea, And He's Also Losing Weight

(Florida)

I got our little guy at 8 weeks of age. I have two other dogs, one is 14 years old, the other is 2 years old, they all eat holistic food, Pinnacle. Yet, after 2 months, the puppy now has diarrhea. He's a happy, active little guy in spite of it, but I'm worried because he's losing weight.

The first time this happened, I put him on a 24 hour fast of no food. (I have done this in the past with the other dogs, even the cats, and that usually solved the problem.) Well, it seemed to work, but then when I put him back on food, by that evening, he had the runs again; so I tried the fast again, and gave him some natural pills I had for the diarrhea (I had used it once on one of the others). Okay, so he seemed fine again; put him back on his foods, and by that evening, AGAIN, it starts, all night! Granted, I know he's a pup, and he eats all kinds of stuff he shouldn't, but I'm beside myself now with concern.

Any suggestions? I'd be wiling to try something, almost anything, but what if it comes back? He's not lethargic or anything, he's his normal, bouncy self, but thinner. Any help would be greatly appreciated.

Thanks