A 3 month old puppy is supposed to have a healthy appetite. Has he been tested for intestinal parasites, or at least dewormed? How often do you feed him? Does he drink and urinate excessively?
Has your vet tested him for diabetes?
You probably should not feed your puppy food for diabetic dogs unless he has been diagnosed.
I found a website that has some information about this.

http://www.dogster.com/lifestyle/is-...mon-in-puppies