How likely is a 75 year old man with preexisting myopathies, who has been unconscious and experiencing muscle atrophy for 6 weeks, to regain muscle if he wakes up in the next few weeks?

Variable: All the factors you describe speak against optimism. Age, duration of disability, and prior muscle disease make recovery difficult. Gentle but focused rehab efforts will maximize what function might be recoverable. Not a quick process!

Coma: For a person in a coma regaining muscle is low priority compared to following commands, speaking, swallowing, and adequate nutrition. Regaining muscle tone and bulk comes later in the rehabilitation process.

Will need PT.: He will need physical therapy to regain muscle even now . Passive exercises during the time he is unconscious will help tone the muscle and he will active physical therapy when he regains consciousness. It may take awhile.