A bulge near the end of the tail sounds like either a spinal injury or infection. Might be a good ideal to get him to a vet. It could be why he isn't eating as well.

20 years is a good age for a snake, not sure how long kings live normally but slowing down of eating may be normal when they reach this age. Even so, good ideal to get him to a vet, especially for the bulge near the end of his tail.