Some pups just need that extra TLC. Have you tried chopping the worms up so that the pup can easily ingest it without having to use extra energy it doesn't have right now? These pups can't compete for food so its best they are partitioned off for a few weeks whilst they build strength and confidence, having other stronger greedier pups around them just intimidates them, they need their own space for a few weeks/ months. Over time the stomach will shrink if they don't get their fair share, its important to offer small amounts as regular as possible, upping the amount over a given time till he or she is eating a decent amount and you can see the abdomen has visibly swelled.
Good luck.