A lot of raw feeders witness a "detox" period that can last a few days to a few weeks, and loose bright yellow stools are part of that. Basically it takes a while to adjust to this type of diet, and I think especially for a puppy their stomachs are even more sensitive. As time goes on, loose stools on a raw diet generally means they aren't getting enough bone. When I had that problem I'd feed legs and wings because they are more bone than meat, you can also add egg shell powder which has calcium like in bones. But since you're just starting out it's probably because he's just not used to it. If he has the runs I would definitely cut back on the amount of food you're giving him and just ease into it. Keep giving the yogurt and maybe some pumpkin although don't overdo it. My dog definitely went through a detox period for a few weeks, I was concerned but it "exploding" wasn't a word I would use so I would take caution but most likely this is normal.