Awww…well…first, just in-joy him every moment! Be present with him, really, really, REALLY, present!
Soak up all his Syness. His beautiful face, eyes, his velvety paws, take in his essence in every moment. :-)
You will know when he is declining and surely you will be keeping up with his vet visits.
Siamese are notoriously long lived, one of mine lived til almost 18!
You can look into having him cremated, most vets will send him out for that service if he is euthanized at their facility, or, you can bury him at home in your own yard.
In the meanwhile, don;t borrow trouble, odds are he has at least several more years.
Watch out for signs of kidney issues, diabetes, and thyroid problems.
These are all common in senior cats and will include extra hunger, more water intake, rough coat, weight loss. The symptoms are the same for all 3 of these very common conditions.
Just love him up, and pay attention to any changes in his heath, behaviors. :-)