Usually if they aren't down by 6 months, then you have a chriptorchid. They surgery for a retained testical is more invasive than regular neuter surgery and should be done, no leaving this dog intact.
There are two schools of thought for when is the best time to neuter. Some people feel with the retained Testical that you should neuter before 2 years of age. Many people are not neutering there dogs until 2, because the hormones help regulate growth. I have not had to deal with that so I am not sure what I would do. It is possible I would opt for surgery now to remove the retained testical and do another surgery after 2 years of age to keep the hormones to regulate growth.
There is increased chance of testicular cancer on the retained one so you want to make sure if it is up there that it gets removed.
This dog should not be bred as this is a genetic trait which will be passed on to the offspring.