Hi Vickie:
17 years! That's awesome! The symptoms you describe of heavy breathing and lethargy are concerning at any age but certainly in his senior status I would have him evaluated today by a vet. Difficulty breathing and lethargy can stem from something serious, such as a heart problem, or it could be a simple infection that can easily be cleared up. Even if it is a heart problem often drugs can be given to provide relief with in a day or so.
I hope it isn't anything too serious, but I would be afraid to wait it out over the weekend.
Dr. Debbie
Veterinarian
Full time practicing companion animal veterinarian.