This is not a supplement, but have you thought about getting a pup? When my first dog, a short-haired pointer, got older we brought in a pup. This may seem cruel, like the old dog is being replaced, but it does have some great benefits.
1) the older dog teaches the pup how to behave; energy level, commands, respect.
2) since it was a hunting breed dog I wanted the older one to teach the pup what animals (pheasants) I was looking for and how to point/retrieve.
3) Most importantly for this thread is my older dog stepped up his energy level to be able to show the younger upstart who's boss. These are animals after all and my pointer was able to hang on to his title as Alpha Male for 2 more years until we lost him.