Oh wow, poor guy! Have you tried giving him any vitamins? PolyViSol infant drops with NO IRON might help him. He is really off balance. It looks like he's really trying but just can't get it together. My first go to thing is vitamins just in case of a vitamin deficiency. SavAChick or nutridrench also have vitamins and electrolytes in them. That's just to see if it helps perk him up some. If it's a neurological issue or even Mareks, I don't know.
Have you checked him over real good for lice or mites? They can drain the energy of a chicken as well.
That's a great, although sad video and hopefully someone can come up with a better idea than I can. I always try to start with the basics because they really can't hurt.