My dad's husky puppy Chugach has been having watery and explosive diarrhea ever since he was taken out of boarding. It's been going on almost three weeks now and what I've tried hasn't helped.

The start of the problem
1.  The boarding place informed us the diarrhea started while he was in their care.
2. Turns out they didn't watch him close enough and he ate so much mulch and rocks that he was drooling and the vet on staff gave Chugach something to help flush his inflamed colon.
3. The day after bringing him home we found mature round worms in his poop so we got him to a vet and started him on medication that he's since finished.

What he eats: Taste of the Wild Puppy Formula
What I've given him: Pumpkin and plain rice

Mind you, the boarding place was given his puppy food that he had been eating at home and he had no problems with it prior to boarding.

After a week of putting a spoon full of pumpkin in his dog food each day there has been no change in his poop. Is there some kind of fiber supplement I can give him?

How long ago did he finish his worm treatment?

A little over a week ago. It was a three day powder medication.