:hugsI am sorry. The only thing you can do when dealing with Marek's is to breed for resistance and keep a closed flock.
De-worming on a regular basis from an early age helps to keep their intestines strong and prevents nutrient deficiencies, that can play a part in how susceptible they become.
How old is he (month, years)?
When did you last deworm him? As good roosters mostly feed the hens with their findings and only eat the leftovers after the hens are done, they are often more affected by worms and can emaciate quite quickly.