Can foods have more calories than dry foods. I would try to mix dry and a can together. Maybe feed twice a day. Once in the morning and later in the evening. Try frying up some ground beef. Adding some beef to the diet may be good as well.
It is summer time and the dog may not eat as much in hot weather. Try a little more exercise at cooler times during the day. Good exercise may induce the need to eat more.
Also check the dog's teeth. My dog Dirtnap had a serious tooth decay issue when first adopted. I had to add water to the dry food to get him to eat. He was about 12 at the time. I also added a teeth cleaning routine with doggy dental wipes.
This should get you started in the right direction. Some of the senior formula dog foods do contain less calories so you may want to keep with a regular or puppy food. Hope this helps!