Aggesssive Lab/Pit mix

by Brittney
(Georgia)

I have a 6 year old Lab/Pit mix (Valet). I got him when he was puppy, so he's only had me as his owner. Never been abused. Well taken care of. He is kind and loving and playful 90% of the time, but he has an extreme fear of thunderstorms, especially if its' lightening really bad (and really loudly) or if he just does not want to go outside.

I live with my grandmother, so if I'm out of town or working she helps take care of him for me. She also has 2 other dogs a 9 year old female Lab (Dutchess) and 7 year old male Long-haired Dachshund (Laddie). Laddie, the dachshund is aggressive towards my dog, Valet. He continuously growls at him, barks at him--to the point where Valet is scared of him. We even had to move where we feed my dog because he was too scared to eat around Laddie which sounds weird for a big dog like Valet to be scared of such a little dog, but its true.
About a year to a year and a half ago Valet started to growl at my grandmother whenever she tried to make him go outside after it had been storming. As his owner, I can usually get him to go outside in these situations. He'll sometimes growl at me, but I try to correct the behavior and then make him go outside. Obviously, what I'm doing is not working because he has been really vicious towards my grandmother twice, and that is two times too many for me. He never bit her, but he did growl and refused to let her near him. After the first incidence of this, she became scared of him, and rightfully so.
I'm wondering how I stop this from happening. Valet has never bit anyone, but I'm afraid that if I don't do something he may. How do I get him to not be so aggressive in these situations and if he starts to growl or show other aggressive behaviors how do I correct him? Also, how do I get Laddie to stop being aggressive towards my dog? Laddie has never bitten a human before, but he has tried to snap at my dog. He never actually bit him though. I don't want to give him away or have him put down because he is my baby. Please help me!