Definitely don't trust dog diapers to prevent pregnancy. The rule is two closed doors between them for 3 to 4 weeks. It really is very difficult and stressful in a home situation to keep them separated. Due to overpopulation and mass euthanasia in shelters, in the U.S. everyone advocates spaying before the first heat since that is the only effective time to prevent mammary cancer and prevents any accidental breedings. Getting the male neutered is an easier operation and could be an option for you since he is older. There is a lot of literature out there on the harm of spaying/neutering early. There is a lot of talk about performing vasectomies and tubal ligations rather than the current testical removal and ovariohysterectomy on dogs. Its common sense that we/they needs these hormones, but even after researching the subject I am not sure where I stand on the issue.
Here is one link. Just do a search "is early spay neuter harmful to dogs" and many will come up and you can try to make an informed decision on what is right for you and your dogs.
Don't Make This Mistake When Scheduling Your Dog's Neutering Procedure