Hi there. Thanks for the question. Let me see if I can help.
Im sorry to hear your dog is not feeling well.
In an older dog, a lot of things could be going on......of course cancer has to be a possibility- stomach or intestinal cancer. Now typically cancer would be more of a slow onset but not always
Another more likely possibility would be pancreatitis. This is an inflammation of the GI tract usually from eating something they shouldn't. It was good you tried the bland chicken for eating but too bad he couldnt keep that down either.
If you can get into the vet soon that would be ideal. They can run some bloodwork and perhaps do an abdominal xray or ultrasound to help diagnose the problem.
If you can not get there soon. Continue to try to feed small meals of plain cooked chicken breast. No dog food or treats.
If this continues for another 12 hours, he needs to get to the vet immediately. Make sure he keeps drinking water.
Best wishes. Let me know if you have any further questions.