Welcome to BYC.
Was he vaccinated for Marek's?
What type of food/treats do you feed him?
Any signs of coughing/sneezing, respiratory issues?
What does his poop look like?
Any swelling of the hocks/joints?
Is he molting or just losing his feathers - any chance he got into anything toxic/rotten?
With legs splayed like that and not being able to get up, my first thoughts would be Marek's or possibly vitamin/nutritional deficiency.
Provide him with some poultry vitamins that contains B2 (riboflavin) in his water or you can crush up a human B-Complex vitamin and add it to his feed. Try to encourage him to drink and stay hydrated. Offer him some extra protein like egg, tuna, mackerel or meat.
Edited to add:
Also you may have placed him newspaper for the photo and your floors look nice/shiny and clean
Consider placing him on a rubber mat or shelf liner so when he tries to move, he has more purchase. Chickens and slick surfaces don't do well together. Check to make sure he doesn't have any leg/hip joints out of place.
Here's some info on Marek's:
https://www.backyardchickens.com/a/the-great-big-giant-mareks-disease-faq
