Have noticed that our Roo has lost his feathers just below his butt. Our hens are just fine and no feather loss. Is this something typical for a Roo this age and since he is sexually active or could something be going on with him?
I would first rule out that there are lice and/or mites. I'll give you a couple of links. The Ohio U document has some good pics to show what to look for, and recommentds Sevin, which many use. The thread talks about alternate approaches.
If you don't find parasites, probably the hens, or one or some of them, are picking the feathers and may be eating them. Sometimes they will do this if they feel a need for protein. It can also result from boredom or overcrowding. Feather picking can turn into a real challenge to stop.