probably your best adviceAs someone who has Marek's in their flock and just had to cull a legbar cock with pretty much identical symptoms, it sounds very much like Marek's disease to me. It was my lad's second attack too and more severe than the first from which he completely recovered a year ago and has been absolutely fine in the interim period. I have been giving him supportive care over the past couple of weeks but when he got to the stage that he was refusing to eat, I knew it was time to end his suffering. My experience of Marek's is that it does come on out of the blue.... fine one day and then floundering the next, I believe there are several different strains that exhibit in different ways, so one person's experience with it may be very different from others. I'm fortunate that mine only seems to affect a relatively small proportion of my young birds whereas others on this forum lose almost all of theirs. Nevertheless, it's still pretty heart breaking.
Good luck with your roo. All I can suggest is that you keep him as comfortable and stress free as possible (stress usually triggers the outbreaks) and consider sending him for a necropsy if he dies or you decide to end his suffering.... or consider doing one yourself if you feel able.
Regards
Barbara