I am so sorry you experieced that with Otis.
Everyone has given you some great advice. There were just a couple of things I wanted to add. You stated this wasn't the first time you grabbed his collar. In dog terms, grabbing their collar is like someone grabbing the front of your shirt and shaking you. It's thier natural instinct to fight that sensation. Luring with treats and making the bathroom more comfortable for Otis is a great idea.
The only other thing I wanted to address are your emotions. Everyone is right, you need to stay calm and in control - a lot easier said than done! Facing a dog, especially your own baby, with his teeth bared is not an easy situation to be in. Take a deep breath and face him as if you were facing one of your kids, no nonsense. Dogs pick up on body language and voice changes much more quickly and accurately that we silly humans. By standing up straight (never lean over your dog when he's upset, they see that as a form of aggression) and speaking firmly and calmly you portray yourself as in control, even if your heart is ready to beat right out of your chest. Keeping him on a leash, may make this easier for both of you. With practice it gets much easier, trust me. I strongly suggest you take Holly's suggestion earlier and talk to your vet. Good Luck!
Sorry this was so long.