This does sound like hypoglycemia, on top of whatever is causing him to have pain in that front leg. Here is more about it:
In terms of treatment, it is REALLY important that you keep calories going in to him. Also, offer lots of clear fluids. So, water is fine, but also he can have pedialyte, Gatorade, apple juice diluted 50:50 with water, or chicken or beef broth diluted 50:50 with water. Give the fluids in small amounts frequently. For a dog this size that means about 1/4 cup an hour.
Given that he is so tiny and has not eaten in days, I would like you to keep putting pancake syrup (or honey, etc) onto his tongue and gums. Just keep putting a drop on every 5 minutes. Also, work on getting fluids into him as above. It may take an hour to get his blood glucose levels up to normal!
Once he is brighter, then you can offer a small amount of his regular dog food, but if he doesn't eat it offer a bland diet. For patients that I see, I recommend a mixture of 75% cooked white rice, and 25% low fat protein. For the protein you could use extra lean ground beef, boiled with the fat scooped off, or chicken breast boiled with fat scooped off or even scrambled egg cooked without fat in the microwave. Feed small frequent meals. For a dog this size, I would suggest 2-3 tablespoons every 1 to 2 hours.
I would strongly recommend that you keep a tube of nutristat on hand at all times in future to prevent/treat this problem if it happens again. Here is more about it: http://www.agri-med.com/site/255063/product/NUTRST-4.25 It is a calorie-dense paste that you can syringe into them to get maximum caloric impact from a given volume of food. It's very good to have on hand for small dogs prone to low blood sugar as it is such a serious problem!
I hope that this helps you!
I do think that you need to get your dog to see an emergency veterinarian as soon as possible today as his condition sounds serious. He sounds very sick and I am worried that it may be dangerous to wait until tomorrow to see your family veterinarian if they are not open today.
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Best wishes to you and your dog!