Now I have to say that I am quite concerned about Harry.
Based on his history, we do have a few concerns here. Now first when we have a dog losing weight, we tend to consider the causes in 3 categories: (1) conditions that cause weight loss due to decreased nutrition intake (one issue here), (2) those due to increased output (again a major issue if he has soft stool, and black stool), and (3) those internal issues that cause weight loss by siphoning nutrition away from the body (ie worms, metabolic diseases like diabetes, organ issues like liver or kidney, cancer, etc). So, in Harry's case, we would have to actually consider possible issues from all 3 categories.
With that all in mind and the fact that we have black stool (which is a sign that we may have an active bleed in his stomach or small intestine --possibly a stomach ulcer or tumor), we really need Harry's owner to get him to his vet urgently. In fact, if they refuse to, then we'd have to consider that this is serious enough to refer to RSPCA (LINK). He cannot be left to starve himself and waste away, as it is cruel for him to have to do so.
Otherwise, if they do have him seen, their vet will be able to have a feel of him to make sure there are no sinister lumps and bumps to blame for his weight loss. As well, bloods would be indicated to check his organ function and potentially further diagnostics if they do suspect an ulcer in the stomach based on the black stool you have noted.
In the meantime though, if his appetite is poor, then consider feeding him wet puppy food. It is 35% water, so it will get some fluid into him if he cannot drink properly. As well, puppy food has more nutrition per bite, so it will allow us to get more into him even with a poor appetite. And if you are seeing him regularly, you can consider starting him on an antacid (ie Pepcid (More Info/Dose @http://www.petplace.com/article/drug-library/library/over-the-counter/famotidine-pepcid) or Zantac (More Info/Dose @ http://www.petplace.com/article/drug-library/library/over-the-counter/ranitidine-hcl-zantac)). These will help with mild nausea that could be triggering his appetite loss. As well, they can reduce stomach acid levels which would be of benefit if this is eroding his stomach wall and causing an ulcer.
As well, to at least help firm up his stools and try to avoid losses from those, I would note that you can put him on a dog safe anti-diarrheal. As I am sure you appreciate, these would not be a cure if the cause were infectious; but it can still be of benefit. It will reduce diarrhea load, allow the body to absorb more water/nutrients, and soothe the upset gut. In regards to safe options for your wee one, the one we most commonly use is Kaolin/Kaopectate (More Info/Dose @ http://www.petplace.com/drug-library/kaolin-and-pectin-kapectolin-k-p/page1.aspx). This is available OTC at most pharmacies. Otherwise, Propectalin, Canikur, Fast Balance, and Protexin Pro-Fiber (which is available OTC at vets, pet stores, and even Amazon) would be another option. All will slow diarrhea and those last ones have the added bonus of providing support to the delicate good GI bacteria.
Finally, without seeing the lump, I cannot really comment on it. If it looks like a discreet fluid filled lump, then it is likely just a cyst. But if its diffuse fluid, then low circulatory proteins secondary to the diarrhea or internal issues with his liver would be suspect.
Overall, in situations Harry sounds to be in a dire state. He needs to be seen urgently, even if that means that the RSCPA need to be involved. As I noted, his weight loss does raise serious concerns here. Therefore, in this case, it would be ideal for them to have him seen by his vet +/- blood sample tested to identify which of the above is to blame and determine if we can help him recover from this.
I hope this information is helpful.
If you need any additional information, do not hesitate to ask!
All the best,
If you have any other questions, please ask me – I’ll be happy to respond. Please remember to rate my service once you have all the information you need. Thank you and hope to see you again soon! : )