My dad who is 78 has had muscle pains for last few months but is now getting worse with quite a bit a pain and feeling very tired with no appetite.

He should be : Examined by a doctor and have some blood work done. His symptoms are non-specific, but could be related to something serious.

Thorough examination: Is needed in diagnosing his problem, by an internist or a gen surgeon. He might be having abdominal problems, or others, which need to be properly evaluated. It could be something very simple or a sinister lesion, but the earlier it can be diagnosed, the better he'll get sooner. Get him seen. Good luck.