[cypher]

It sounds as though it may just be old age.
The heavy breathing is a bit of a concern, it may be that he's reaching his time to go to the bridge now but if he continues to seem healthy other than the breathing & lack of activity then it may be worth a vet check just in case he has a respiratory problem.
Even if his teeth are ok he may not want his normal food so much so you could try offering some soft foods in addition, soaked mix, porridge, scrambled egg & safe baby foods are all good to try.
At this age they often become much less active & just want to sleep most of the time, make sure food & water are within easy reach of where he's sleeping.
Keep him warm too, lots of shredded toilet tissue to nest in, even if he seems to prefer sleeping in the open give lots.