How to help a small dog with aggression issues?

My mother has a 4 chihuahuas. The 4 year old unfixed male chihuahua is extremely aggressive. It is possessive over the female chihuahua. It dominant humps the other dogs in the household, including its son and daughter. It is also unnecessarily protective of the other chihuahuas. If you are petting the other chihuahuas and they are fine with it but he gets aggressive. He will growl, bare teeth, bark, and nip. I have the pleasure of watching this dog for a week while my mother is on vacation, along with the son chihuahua. The son seems to have developed some of the same traits as the father, but on a lesser scale. He is generally accepting of petting, and is usually friendly. But he is skittish around other dogs, and he fear bites. I am a very strong advocate of positive renforcement. I use clicker training to train my dogs. My dogs are wonderful. They listen, they know a handful of tricks, they are sociable and are well socialized, with people and animals. So I figured I would give positive renforcement and clicker training a try. But the dog will not accept any treats from me. It turns its nose up at anything I give to it, its own kibble, chicken flavoured dog treats, and even peanut butter wasn't enticing enough. So since I figured I would have a difficult time with clicker training, I just wanted things to run smoothly. So he has a schedule now, for going pee, food, and walks. I wanted to include training/playing in the schedule, but he won't play. He won't chase or tug toys, he doesn't play 'tag' like my dogs do. He acts like he is going to be attacked if I try to entice him with a rope toy. And when I bring him back in from outside, mind you it is slushy/snowy outside, I lead him to his crate so he doesn't track mud everywhere. He growls at me when pointed to his crate. He even nipped at fingers, and bite my shoe (two separate occasions). I called my mom about his behaviour. She says it's "normal" for him to act like that. I strongly disagree. I think his behaviour should be curbed somehow. Any suggestions? My boyfriend has been biten 4 times by them already, I've been luckier as they never made contact on my skin. The two dogs together act like they are a team against the world. Everyone is an enemy. I try to feed, play or train them, and one or both will growl at me, 'defending' themselves or each other. They act like their entire world is coming to an end if they are separated. Yet they will fight like mad with each other when together. They growl and snap at each other if one is too close or doesn't something else that angers them.

Your mum just loves the dog that's why she's saying it's normal. It's most definitely NOT normal for a dog to growl and bite people. I volunteer at an animal shelter with the dogs and if there was any kind of behaviour like this the dog would have serious training sessions to get it out of such bad habits. It sounds to me like he's spoilt, he's got too much of a free reign. He thinks he's the boss of the humans AND the other dogs because your mum lets him get away with it. A sign of a dog thinking it's the dominant one is humping the other dogs like you said he did.The only reason that dog has been getting away with it and not scaring your mum to death is because of it's size I imagine. If he was a rottweiler and he was biting hands and growling, it would be a different story. I suggest if you're up for it you can try and train him, otherwise classes might benefit him. You need to just let him know he's not the boss. Kudos for training your own dogs.