He may be resting/rubbing on that area while roosting, causing the feather loss. I've had birds were the exposed skin was very red just from the exposure to sunlight (seems less likely on his underside). I can't really tell where exactly the nodule looking skin is...so not sure about that, could be from inflammation/irritation, or as previously suggested, possibly tumorous. Very hard to determine that from photo's. I also would treat like dermatitis, and check him while roosting. Padding the roost where he goes may help. Just make it something they can't peck pieces off of and swallow. An old bath towel wrapped around the roost can work.
