Hello,
This is Dr. Christian. Welcome to JustAnswer. It is fairly common for a mature male dog to have some post-operative bleeding into the scrotum because of the increased blood vessels in the area compared to a younger dog. The best option is actually cold compresses because the cold causes the blood vessels to constrict and can help close bleeders. The warm compress actually causes the blood vessels to dilate and worsen the problem. As long as you keep the area clean and his gums are nice and pink then he'll probably be just fine. I would recommend having it checked in the morning by a vet as soon as you can. In some cases the amount of blood in the scrotum can lead to infection or other problems but those won't be of concern tonight. If you have more questions please let me know.
Dog Veterinarian
12 years of veterinary experience in canine surgery, medicine and behavior
You are very welcome. Good luck.