I'd get him back to the vet as the sickness may be parvo or distemper and he may need more treatment. If it is parvo ask the vet about using Tamiflu the human med which is helping save pups across the country.
www.marvistavet.com/html/body_oseltamivir.html
This might also be kennel cough
www.marvistavet.com/html/kennel_cough.html or if he is really sick the new dog flu that has been in the news.
The leg may be an issue with a bad knee or with a hip problem if its a rear leg. If its a front leg then it might be nutrition or an elbow or shoulder problem. If its a nutrition issue getting him well and eating right should help.
Any Jack Russell that is described as listless and any puppy who has no interest in food when he's not full is indicating he is pretty sick.
You should at least call the vet and tell them he is getting worse not better and ask if they want to check him for parvo or distemper.
Hope this helps you!
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