Simply the stress of the new environment can set off diarrhea in the puppy -- ask me how I know.... Kaiser had either mushy or liquid poo for his first week with me. I do agree that canned food is not helping the cause.
My 'secret weapon' is canned pumpkin. Not pie filling, just plain canned pumpkin. I am still feeding it to Kaiser twice a day to ensure that things stay on the firm side. I also occasionally feed plain cultured yogurt to boost the gut flora.
Decreasing food won't do any good -- you can withhold food entirely until the diarrhea stops and then start to introduce it again, but I don't think that's necessary in this case (since it was likely brought on by switching the food, not a stomach bug). I'd put him back on the solid food, too. Some puppies pick at their food (mine does); they won't let themselves starve to death. He'll learn to eat.

Luke -- Lab/Shepherd (1-1-04)
Kaiser -- Alaskan Klee Kai (12-30-07)
Sar -- Arabian gelding (4-11-80)
Norman -- Percheron/TB gelding (6-18-00)