I really think that I would make him comfortable and go from there. If he is suffering, I would let him go. Are you still thinking about a necropsy or have you called and found out how much they charge?
Mareks is a tragic disease. Getting a diagnosis would be helpful to know how to deal with this. There are 4 types of symptoms—the eye or ocular form, the skin form, the nerve form, and the visceral form that causes tumors in organs. Mareks causes weakened immunity to common diseases.
Since this is the second bird within a week that might have Mareks, you really should not take birds to shows. If you have Mareks, that may be where it came from, unfortunately.