Large growth on rooster leg
Looks to me like either a): Scaly leg mites or b): Bumblefoot. Bumblefoot is always on the feet.
Are the scales raised (so you can see space between them)? Or is there a large welt-like thing?
If its the first one then you have Scaly leg mites. Check the other chickens for raised scales too. If they don't have raised scales, then it may not be scaly leg (But I'd still assume it is).
Treatment (commonly reccomended)
1: Dip rooster's legs in warm water
2: Gently brush off with a toothbrush and a bit of white vinegar.
3: Rub either vaseline, olive oil wth. Cayenne, Coconut Oil, or a Vaseline Substitute (Waxeline) on the chicken's legs. Administer the same treatment to the rest of the chickens.
Could the mites cause a large mass to grow from the back of the leg? The pictures don't show it well, but the scaly looking part you see is actually a mass that is growing from the back of his leg. My first thought was that maybe he got some fungus in or around his spur on that leg, because the mass has grown into a blunted point and has started to curve downwards. I'll try to get another picture. Thank you so much for your quick responses!