Yeah very rare for Mareks disease to ever be a problem in broilers for that reason.
Sounds like a well bred broiler. Gained weight so quickly that it couldn't even move.
Any pictures? A Cornish cross or red ranger id presume?
At 6 weeks I would process him. If you goal is a standard 8 weeks, you won't gain much by Nursing him for another two.
The feather development on broilers is ideally slow or non existent. Less to pluck.
Make sure to take a good look...