Hi Charles, This is Dr Weaver, I am an experienced equine vet. I am sorry to hear that Jake is losing weight.
A few questions for you, what type of feed and hay are you currently feeding Jake, and how much?
Do you know when he had his last dental float?
The first thing I think about in an older horse that is losing weight, is dentition. Horses at that age may have worn their teeth down, or worn them unevenly, and may not be chewing their food as effectively. A through oral exam by your veterinarian, and dental floatation to remove sharp points is important to keep him comfortable. I recommend yearly oral exams and dental floatations.
I also recommend feeding a senior feed to horses older than age 25. If his teeth are very worn, he may need to go on soaked hay cubes or hay pellets as well. You can safely feed a lot of senior feed to older horses. Usually I recommend feeding about 3% of his body weight per day in senior feed and hay, so for a 1000lb horse that is about 30lbs of feed a day. So he could get 7-8lbs senior feed and 7-8 lbs hay/soaked hay cubes or pellets twice a day.
Yes cancer is always a concern in an older horse losing weight. As well as liver/kidney problems, and heart/lung problems. I would have your vet out to do a through physical exam, check his teeth, listen to his heart and lungs, and perform basic bloodwork to rule out infections, cancers, and underlying organ dysfunction. He may even want to do a rectal exam to feel for abdominal masses.
Let me know about his current diet, and I hope the above information is helpful.
-Dr Emily Weaver