I would also recommend having a Vet do tests to find out the cause.. Your horse is considered geriatric and careful monitoring in general would be needed in this case, but since he has had a history of cancer in addition to his age, extra care would be recommended.
Let us know what the results are.
I would consider getting your vet to do a liver enzyme test, it sounds like he has jaundice which can be a sign of an ailing liver, or a liver condition. Possible that the cancer could have spread.