Golden Puppy being spiteful? Refusing to fetch?

Question
I have a male purebred golden puppy that is 5 months old. He's quite big for his age, and most people who see him think that he is an adult dog. He's about 55+ pounds and is very sweet. Our household is made up of 3 kids, me being the oldest [17], my two younger sisters [2 & 12], my mother, and my declawed male cat [5]. As far as I've noticed, the puppy is gentle with everyone in the household except for the 12 year old. He plays with her like with nobody else, and even though it starts out as playful, he eventually jumps on her, starts growling, biting, and humping. He doesn't do this with anyone else in the family, not even the toddler or the cat, who both have either kicked him [she's young..] or hissed/pawed him in the face [my cat doesn't like my dog's attempts to play with him]. We've tried to clamp him mouth shut, say "NO", clicking on the nose, etc etc, and nothing works. We don't know what to do because he is obviously going to grow into a VERY large dog, being almost 60 pounds at only 5 months.

Another concern we have with him is housebreaking. We all have a busy scheduale, with school and work,  etc etc, but he's managed to adjust to it Monday-Friday. I'm sure that he can handle holding it in, since I leave at 7am and come back at 1pm every school day, and he is able to hold it that long. However, on Saturdays, when he hears the people are home, he will deficate and urinate, more than once, before 9am, when my mother takes him out at 5:30am. She thinks that he is doing it out of spite, as a dominant type of thing, though I wouldn't understand why he'd do that.

On top of that, he refuses to eat unless I sit with him [most of the time]. The vet said that he is underweight for his size, and when he put food down for him, even in much smaller portions that he is supposed to recieve, he doesn't even touch it half the time.

Otherwise, he is a wonderful dog, we taught him to sit. However, he refuses to fetch. When I throw him his toys, he loves to grab them and wait for me to come get him, then he backs away and runs around, expecting me to chase him. He is a retreiver, he's supposed to retreive..

Sorry for the long bunch of questions, I hope you can help. Thanks a lot!

Answer
Sounds like a classic case of the dog being in charge.  The ideal solution would be for the 12 year old to take it to a 4-H dog obedience club.  Unfortunately it may be too late for this year.  In my area, clubs form soon after the first of the year.  Even many urban areas have 4-H.  For info look in your phone book under government listings for extension or cooperative extension offices.  Ask specifically about a dog or canine club.  The dogs see all the people and dogs in the household as a pack with each having their own rank in the pack and a top dog.  Life is much easier if the 2 legged pack members outrank the 4 legged ones.  You can learn to play the role of top dog by reading some books or going to a good obedience class.  A good obedience class or book is about you being top dog, not about rewarding standard commands a treat. Start at http://www.dogsbestfriend.com/

Somebody must claim the top dog role and train the doing that all the rest of the people outrank it.  Even if the 12 year old isn't the one, she must spend time working the dog with its commands.

Here are some other ways of exerting leadership:

''Elevation for small puppies:  Sit on the floor and gently put your hands around your pup's middle, below his front legs, and lift him up.  He is facing you.  Hold him for 15 seconds.  Repeat until he no longer struggles.  If he is past 10-12 weeks, lift his front feet off the ground, but don't pick him up.  

Cradling for small puppies:  Hold your puppy gently on his back, as you would cradle a small baby.  If he struggles, hold him firmly until he quiets for 10-15 seconds.  With larger pups, you can do this as your sit on the floor, with your pup between your legs.

Quiet lying down:  Place your pup on the floor on his side, with all 4 legs pointing away from you.  Use your hands on his neck/shoulder area and middle, to hold him in this position.  When he is quiet, praise him.  Lengthen the time that you keep him quietly in this position.  When he accepts this position well, handle his paws and muzzle, while keeping him quiet.''

The quotes mean this isn't my original work.  It is copied from my Puppy Raising Manual.  I have long used these or minor variations of them, and they are very effective.  You may want to give him a belly rub while he is on his back too.  Helps bonding.  There is a big difference between him rolling over and demanding a belly rub, and you choosing a time to roll him over and rub his belly.  The latter cements your place as pack leader.

Have you ever watched a couple of young dogs play?  One will grab something, rub it is the face of the other, and run off wish it with the second running after it.  That is exactly what he is trying to entice you into doing.  As you take control through the above, he should start to obey come, and then give or drop.  You can't expect him to retrieve until you out rank him.  

Vets that don't give good feeding advice are more common than healthy dogs that don't eat as much as they need.  Can you actually see his ribs?  Measure him and your vet's advice against http://www.puppychow.com/products/popup_body_condition.aspx  There are exceptions.  Most Labs and Goldens are highly food motivated, eating more than is good for them and looking for more.  At 5 months his growth should be slowing downing he should have 80-90% of his adult size.  If he is really under weight, I guess continue puppy chow switching to a concentrated, meat based one such as the Pro Plan I feed if not already.  If you change food do so slowly over a week to avoid digestive upsets.

I don't think you will solve any of the problems until he is demoted from top dog.  As he finds his rightful spot in the pack, most of them should go away.  If you are still having problems in a month or 2, get back with the details, and I will make more specific suggestions.