It looks like s/he is being picked on or it's a mounting injury.
Can you post a photo of the whole chicken, are you sure this is a rooster?
Is the other chicken s/he is with older or another rooster?
How are they being housed - are they outside in a coop and run - how much room do they have?
You may want to separate the two so s/he can heal.
If you can get it, gently clean the area with some saline or betadine, then apply some Vetericyn or plain neosporin. If s/he is outside, make sure flies are not getting on the wound.
Feed normal feed, you can add extra protein by offering eggs, tuna, mackerel or meat. Watch for any signs of infection - pus, oozing sores that smell, etc.
Also check to make sure there are no mites/lice, look around the vent for any critters.