The diarrhea could be a result of many things: e.g. stress related to any injured limb as you've indicated, dietary indiscretion or intolerance, intestinal parasites, gastroenteritis, pancreatitis, other systemic diseases, etc.... there are many causes.
I'd suggest a bland home-cooked diet of chicken and rice to help with the diarrhea. A small amount of canned pumpkin can be added to his food to increase the fiber content and help bulk up his stools.
It's impossible to say why he won't use his legs without seeing him in person, and even then often times diagnostics such as imaging (x-rays) are often necessary to determine the issue. It's possible the reluctance to walk / lameness are related, it's also possible they are completely separate issues. Did you see an injury occur?
My overall suggestion would be to keep his diet bland to help with his diarrhea until you can get him seen by your local veterinarian about the leg +/- the diarrhea (if it's stress related a simple bland diet may be all that's necessary to help it resolve). Depending on the physical examination, some outpatient antidiarrheal medications and pain medications for the leg might be tried. Or, you might be looking at diagnostics such as a fecal parasite check, x-rays of his limb or spine, labwork...etc.
Keep his diet bland, rest him, and get him to be evaluated in person to determine the next best step as soon as you can.