It depends on the breed, to some extent. In some breeds, such as Dobermans, the females tend to be the dominant personality, in others, like Bull Mastiffs, males. It can also be a matter of personality of each individual dog. I currently have two female dogs and one male. My female Collie-mix stops to pee a LOT more than my male Aussie - I think she's just a more dominant, territorial personality. My other female, a terrier, is the most gentle, lazy dog you could ever meet - really very opposite to my outgoing and active Collie. My male is just a happy-go-lucky guy. Instead of choosing a gender, I would look for a personality that you like.
But if I could add my two cents, please consider a rescue dog. You can still get a puppy if that is important to you, and while you might not have as much choice on gender, you'll be saving a life and creating a lifelong bond with an animal that truly NEEDS you. Please visit petfinder.com to find dogs in your area - you can search for a specific sex, breed, and age, or just put in your zip code and see what is out there. I am the proud mommy of three rescued dogs and two rescued cats after growing up with breeder dogs my entire childhood, and I have been working with animal rescues (both shelters and private rescues) for the last 10 years. Let me assure you that you'll never find a more loyal family member than one that has been saved. Good luck.
EDIT: One more thing I forgot to add! As long as you get the dog fixed, it shouldn't matter that you are female or your sons male. Fixing a dog will suppress those hormones.