Can you get a vet out to examine him and have that include talking about stomach ulcers and hind gut ulcers.
It doesn't matter how a cushy a life he might be living, he's lived long enough for his digestive tract to possibly have run amuk and maybe developed ulcers, which prevent him from utilizing his food. I would especially worry about that if you see a lot of whole grains or hay in his manure.
I would also talk to the vet about upping his protein and his amino acids; something most older horses can benefit from.
I see you are in Sweden so I have no idea of the things available to you.
My Arab never did have a lot of muscle. He's now 26 and has even less.
I feed him 1 to 2 pounds of well soaked timothy/alfalfa cubes daily. Soaked because he has four molars missing and the alfalfa is a great source of protein and amino acids.
He deals with mild gastric ulcers. He doesn't get any sort of grain but is on a vitamin/mineral supplement that has a pre-probiotic in it to help with digestion. There's some rice bran mixed in to help with fat calories
He gets two teaspoons of Brewer's yeast daily to also help with digestion. I buy equine Brewer's Yeast in the form of Diamond V-YC.
He's only 13.3H but gets as much hay as my Walking Horses when he comes in each night and he eats all of it.
With all of that, he is still what I call "racehorse ribby" and his hip bones are a tad higher than I want to see. His coat is slick, and his hooves shine. The vet gives him a physical twice a year and says he's doing great.
Anything can happen and diseases can develop at any age but when a horse reaches their mid-20's, they become more susceptible to things and are wearing out, just as we do in old age. Some at different rates than others, just like people.
That's why I think it's important for the vet to give your horse a good physical and maybe add a blood work up to that. Although if he passes his physical with flying colors, it would be yours and your vet's discretion whether or not to do a CBC on him
Hope this helps