I'm so sorry he didn't make it.
As someone who has Marek's in their flock, I would say that his behaviour and lack of coordination in the video was symptomatic of the virus. Often they don't suffer paralysis so much as apparent numbness and an inability to coordinate their limbs probably because they can't feel them. I've seen birds stand on and trip over their own toes with it. It is quite common for them to tip forward but some go backwards and some walk on their hocks.
If you do not want to send his body off for a necropsy and you are not squeamish, you could cut him open yourself and check all his organs and see if you can find anything amiss. If you can take photos of the body cavity and the organs, we have threads where we share such photos and discuss possible diagnosis here on BYC. Sometimes with Marek's you will find a tumour on the lung or heart or perhaps the liver or sometimes there is a mass in the gut. The first one of mine that I ended up euthanizing had a subcutaneous tumour on her abdomen and another on her leg but her organs were OK. Unfortunately she was so badly contorted with paralysis that she could not eat or drink and I felt that it was kinder to end it for her. Since then, I have lost several to it but I've also had a few pretty miraculous recoveries too. Unfortunately those that recover are prone to secondary and more severe and prolonged attacks which can be weeks months or even years later. They occur at times of stress. Perhaps this guy had been carrying the virus for a while and the young male starting to challenge him, triggered the attack, if it is indeed Marek's. It would be interesting to know what his body condition was like when he died? Did he exhibit any respiratory issues during those last couple of days?
