Though male rats can be fertile as early as 6 weeks, it's not a good idea to breed them at this age. Not only are they much smaller than the female you are wanting them to breed and they may be injured in the breeding process, but it's always a good idea to wait for 4 months or even more with a male, so you can get an idea of his personality and health and figure out if you even want to breed him.
Male Reproductive System
The reproductive system of a male is cared for when the rest of his body is cared for, with a proper diet, exercise, and social interaction with other rats and humans. Rats you are planning to breed should especially be taken care of, as both they and their babies to come will require lots of proper nutrition. Make sure your rats are getting a balanced diet with lots of fresh vegetables and fruits added. Male rat should never be fed orange juice, as it contains a substance which causes cancer in male rats.
Reproductive Health Concerns
Unneutered male rats are prone to testicular tumors. If you notice any small lumps on your male's testes, take him to a veterinarian right away. Most testicular tumors are benign and can be easily removed without trouble. However, once a rat has a tumor, chances are it will have more later on in life. Also, a rat that has been diagnosed with a tumor should probably not be bred from, as this can be passed down to future generations.
Though rare, male rats can also get bladder infections. Talk to your veterinarian if you notice blood in the urine, or notice that your rat is in pain while urinating, or unable to urinate.
