Have you had senior blood work done to make sure there's not a physical problem? Are his gums, teeth, tongue and jaw healthy? (Often mouth pain of some kind leads to changes in eating habits - they find certain foods easier to chew and swallow.)
If he is healthy, age may just have made him a picky eater, although IME 14-15 isn't that old. Mine have generally been 17-18 before I have problems getting them to eat.
As the veterinary specialist who has treated a number of my animals who've had chronic conditions once told me - keeping them eating is job one. If you have to give them less than optimally healthy food to keep them eating, so be it.
The things that have worked best for me with my very senior citizens is mixing in baby food (stage one, any of the meats). All of mine have turned up their noses at Beechnut, and have insisted on Gerber. Some of mine have had their appetites piqued by mixing clam juice into their food. Several of mine have spent the last year or so of their lives just on baby food.
Also, with my older ones, I've found that they just need more variety to keep their appetites stimulated.