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1- Is the lump on the shaft of the penis?
2- Can you post a picture of the lump?
3- How long has he had this lump?
So the lump was on the shaft, correct?
Friend, I believe the lump you were seeing is erectile tissue found in the penis of dogs called "bulbus glandis." This tissue becomes larger when the dog has an erection; basically, your dog had an erection. If the glans penis is out it should go back in once the erection is gone. If he can't retract the penis back into the prepuce you can try to put it back in by lubricating the penis with ky jelly or vaseline and pull the prepuce back into the penis. If you can not put the penis back into the prepuce I would have him checked by ER veterinarian, the penis blood supply could be cut off by compression of the preputial opening, that would be an emergency as the penis can suffer irreversible damage.