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Hi, I'm Dr. Barbara, and will be happy to help you with your situation today. It is normal for castrated males to be able to have erections at times, or at least to become sexually excited and display humping behavior. We think that this behavior can also be part of normal play. . .it is an alpha dog behavior. However, it isn't normal for Mopsy to have this bloody discharge from his penis, and it isn't normal for unintentional weight loss. Most dogs with arthritis gain weight from decreased mobility.
I think it would be best to have Mopsy thoroughly checked there by your vet to determine the cause of these two problems.