You should look into clicker training. Loki has fear issues with big dogs and certain other things that make him reactive. Thanks to classes with a wonderful trainer, we have been able to manage his issues and improve his reactivity drastically. I know what sets him off, how to deal with it, and what to do to help him improve. He is just a doll at home, and fairly well behaved when I take him out. The trick is management. I would sign up with a clicker training or positive reinforcement trainer in your area. If you prefer books, let me know and I can recommend some.
Don't try "dominating" him or yelling or anything like that. It won't work.
Think about it. He is afraid of big dogs. What if, every time he barks at a big dog, you hit him? This confirms that big dogs make bad things happen.
OR
Every time he sees a big dog, you pull out yummy chicken. He soon learns that big dogs are OK because mom gives me chicken only when I see one. You have to be consistent and SET THIS UP to happen. That's why you need a trainer.
WHICH would you prefer??
Also, he may have some boundary or other leadership issues as well. Again, don't think you need to be ALPHA, but he does need to learn snapping is not acceptable. When he snaps, gently give him a time out in a bedroom or crate and IGNORE him for 5 minutes. If he persists, immediately give him another time out. It is tedious, but he will learn. Require him to sit nicely for petting, food, getting his leash on for a walk, etc. He needs to learn to look to you for direction so that he does not take matters into his own hands (like mine does, with barking at the scary big dog - often I just go look out the window and say "Thank you" and distract him with a toy or treat.)