Agreed.
He is beautiful - I'm so sorry he is not doing well.
It does look neurological to me too. Even though he is 4yrs old, I would be inclined it could be Marek's...BUT you would still want to try to rule out other things that can mimic Marek's.
https://www.backyardchickens.com/articles/the-great-big-giant-mareks-disease-faq.66077/
Have a fecal float to rule out worms and coccidiosis.
Offer vitamins with B's, E and Selenium (you can use human B-Complex vitamins, no iron if needed) most poultry vitamins have E and Selenium is found in egg, tuna and nuts.
Check him over well for any external parasites and treat with a Permethrin based poultry dust/spray if you find any.
Look in his ears - make sure there is no infection or debris (chickens can get ear mites) - ear infections can cause loss of balance.
Has he been in a fight with another rooster - any possibility that he suffered head trauma?
Any possibility he got into anything rotten/moldy?
You can try placing him in a sling to keep him more upright, but he would need to be monitored. Some take to a sling very well, some freak out. Here's an example below, but if you google "chicken sling" you will find tons of images to get creative.
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