If you want to lower the risk of your dog developing a potentially fatal disease, you need to neuter him. Neutering your dog completely eliminates his risk of developing testicular cancer later in life. The behavioral signs you'll see from an unneutered dog will in most cases disappear after he's been snipped.
Health and Behavior
Neutering prevents testicular cancer and the gradual enlargement of the prostate with age, which can create health complications. Behavioral signs that a dog needs to be neutered include aggression, and neutering lowers the levels of testosterone that influence aggression. Roaming is another behavior of intact males looking for mates, and it usually abates after neutering. This also applies to urine-marking, which is a behavior of primarily intact dogs for communicating virility to potential mates. Neutering is not an instant cure for bad behavior, though. The ASPCA recommends a combination of neutering and training to correct problem behavior.