It doesn't look like the joint to me.. not seeing anything on the toes/ankles.. looks to be above the feather line.. meaning either lost some feathers or possibly dealing with mites/lice.. Have you checked the skin color below his vent and on his abdomen?
Is that redness, above, below, or all around the joint.. I admit my eyes are terrible and photos don't always tell the whole story.
It does not look like normal hormonal flushing usually does.
What is his age? How many flock members, usual feed routine. free range?
Adding your general location to your profile can help folks make their best suggestions possible at glance.. they might know if you have the same resources available or not, etc.. since we are gathered from around the world in this community here..
Hope you are enjoying BYC by the way, welcome!