It does look like Mareks unfortunately. I don’t see why you cannot try vitamins, but I would probably use 1/2 tablet daily of vitamin B complex in food instead of NutriDrench or something in the water, since electrolytes can give them diarrhea if used more than a day or two.
Have you tried using a chicken sling, chair or nest with him? It is a good way to get them upright and out of their droppings, as well as in front of food and water. Here are 4 pages of various homemade chicken slings:
https://www.backyardchickens.com/threads/versions-of-chick-chairs-please.1166308/