Now it does sound like you have been very through and I am glad to hear that those tests are pending, as checking these are indicated here. This is because it sounds like instead of a more common infectious cause (ie pathogenic bacteria, viruses, protozoa, parasites); we have a maldigestion issue (ie exocrine pancreatic insufficiency, IBD, GI lymphoma, etc) or a possible bacterial overgrowth (small intestinal bacterial overgrowth, aka SIBO) with Casper.
Now I do think those results will direct how to approach this for Casper. Further to this, we may need to keep GI scoping in mind as a next step to obtain biopsies and rule out IBD or lymphoma (less likely at his age but possible). Though in the meantime, you could consider speaking to his vet abut supplementing Vitamin B (usually via injection) and considering trying him on an easy to digest diet (ie Hills I/D, Royal Canin Gastrointestinal or Sensitivity Control), a calorie dense diet (ie Hills A/D, Royal Canin Recovery) so we get some into him despite the diarrhea, or even a sterile protein diet like Hill's Z/D (More Info). As well, since you did see that previous bacteria overgrowth, you may want to consider trying him on probiotics. These can help maintain a normal gut population and may be ideal if one species is overgrowing among the others. As well, you can obtain canine probiotics in combination with Kaopectate in OTC preparations like Propectalin, Fast Balance, Prokolin, or Canikur. So, these could be used as well to address both angles at once.
Overall, his signs and test results thus far are more supportive of gut malabsorption or compromise from one of the above. Therefore, it sounds like you are on the right track but I would suggest the above while awaiting those results and do consider the Z/D as a means of getting protein into him without triggering his gut/malabsorption issues.
All the best for Casper,