I'll have to respectfully disagree with our esteemed global moderator and say that it is possible to overdose a dog or puppy with de-worming medications. Other possible bacterial/parasitic problems he might be experiencing are coccidia or giardia. A vet check out might be in order to rule out those possibilities.
At twelve weeks a corgi puppy should weigh in the ballpark of twelve pounds, give or take a few ounces or pounds. Depending upon the overall proportions, amount of bone, general balance of a puppy, one could weigh a few pounds less or even a couple of pounds more.
There is NO one right food or way someone can suggest to feed your puppy. What can be shared is what has worked for us and our experiences and offer recommendations of things that have worked for us. IF you will read back in the archives of the forum, you will find some very heated discussions on this topic. You will also find some very good links to articles and websites on different foods and feeding posted by Darci (Cindy). I suggest looking those up to help with your label reading - an excellent first move on your part <IMHO>.
Not every food, no matter how excellent or highly recommended is going to work for every puppy or dog. A favorite food of many, many corgi owners in the US I found to be too rich for my corgis; stools were loose and frequent; they just didn't do well on it. But just because mine didn't do well on it doesn't mean it isn't a good food. I'm another one who prefers to have meat rather than corn as a first ingredient and I'd much rather have chicken meal rather than chicken by-products in the ingredient list.
The one correct way to feed your corgi is whatever food and regime that works for the two of you.
Debbie