Sounds like your doggy has decided that human food and wet dog food is tastier than dry food. At his age, and considering his other illness, aside from giving him too much human food, I would give him what he likes. Human food is bad for him due to high fat and salt contents but there is nothing wrong with giving it in small amounts or mixing wet food with his dry as long as he gets some dry food as it is good for his teeth. You can also add boiled chicken (no bones) or hamburger and rice to his food to get him to eat it. This can also help with diarrhea. His low energy is likely a combination of the heat and his age. Just make sure he is drinking plenty of water. He sounds like he is ok, I would monitor him pretty closely because of the chicken bones. If you see a dramatic change in his symptoms or if he starts vomiting or become lethargic (not just slowing down) then you should call the vet or take him in ASAP.
Hope this helps!