I would not recommend giving the dog anything sugary. The reason I say this is because lethargy sometimes is one of the symptoms of the onset of diabetes. Has your dog increased its water consumption and urination? If so, then diabetes is a possibility.
I recommend that you bring your dog to your veterinarian as soon as possible. If, indeed, your dog is diagnosed with diabetes, it can be managed with a little patience and a few lessons in checking sugar levels and administering insulin. It's not hard ... we did it for many years for one of our dogs.
It's great that you are noticing behavioral changes with your dog. Those are often the best clues to let you know that something is going on with his or her health. Good luck and the vet, and I wish you all the best!