I am sure you know that Pekins were bred as meat birds grow supper fast to be slaughtered at around 12 weeks old. I don't think anyone thought they would become so popular for back yard duck lovers but they have. So we have to be really careful not to over feed them especially things like bread that will add to their weight but isn't very nutritious for them. Rouens are dual purpose and can probably get over weight also but not like a Pekin. and Probably a Jumbo would need to be watched even closer. Their legs can also suffer from being over weight since ducks legs are not the strongest anyway. Feed probably had nothing to do with it. Many feed their flocks layer. Could have just been something internal that was slow to show up. I had a Muscovy duckling[female] who never would go an forage would rather lay around instead of being with the other ducklings. I thought maybe her heart wasn't strong. She lived to be 4 months old. I didn't do a necropsy so I can only guess what she died of. But saying this because sometimes it is visual they have a problem sometimes not until too late. We all can only speculate on what killed your drake. Hopefully it was just with him though and the others will continue to thrive.
Activated charcoal has been the best for upset tummy. Great to have for humans too.
