Here are some factors to consider when deciding how much to feed your dog:
- size/weight
- activity level
- breed
Whatever kibble you're feeding your dog will have a recommended amount based on weight/size printed right on the bag. Use that as a rule of thumb and adjust it to your dog's own eating & exercising habits. Most people tend to OVERFEED their dogs, which can lead to some health complications later on, particularly in the toy breeds. For instance, my puppy is a chihuahua and she weights in at a mere 2.5 pounds. The recommended daily allowance for her is about 1/2 cup of food, total for the day. That's not much at all -- about 50 pieces of mini kibble. I feed her twice daily, 1/4 a cup at a time -- a TEENY amount.
I don't suggest leaving food around. Feed your puppy 2 -3 times daily, on a regular time basis. This helps you to recognize his eating patterns and tell when he's not eating well or if his diet needs adjusting. Also, food is a primal motivator in dogs and should be used to bond with them.
If you are feeding him on a regular basis, and he JUST started to nibble at his food, I would be concerned about his health. His disinterest in food all of a sudden may be an indication that he's not feeling well. This is assuming you don't leave food out and he's not being overfed. If this is the case, you should monitor his bowels closely and consult with your vet.
If he's a puppy, he may be going through a particularly bad phase of teething as well. Many puppies lose interest in food for a short period when they're going through that.
If none of these is the case, you may want to mix wet food with his dry food. A tiny amount of dog gravy or wet food goes a long way, and most dogs gobble it up. Mine licks her bowl clean!
Good luck and let us know what happens!