I love my dog, but...

He bit me today. He's normally a pretty good dog. Some of his antics can be annoying, but his behavior with people and other dogs is friendly and playful. He goes to the kennel and is in doggie daycare and plays all day long with other dogs and is fine. We have been in our home and other people's homes with other dogs and he is fine. The problem is when he's on his leash and sees another dog and the other dog starts barking at him. When that happens - he goes nuts and is hard to control. When A and I are walking him together, we can sometimes avoid the unpleasantness by distracting him - breaking the cycle. We do that sometimes with treats. Today on our walk, first dog to bark at him was in a yard. Boomer cried, but then nudged me as if to say "I'm being good, give me a treat." He was being very good, I told him so and gave him a treat. I thought we had done it, trained him to be good when other dogs barked at him in exchange for a reward. (Boomer is very food-motivated.) We were further in our walk, almost home when we see a dachsund on a leash. Dachsund starts barking. I try to distract Boomer with a treat. That does not work. I try grabbing the scruff of his neck he twists away and bites me. He didn't break skin, but I think that's only because I had a winter jacket on. He actually left a mark. I was so upset, I did something I should not have done, but I spanked him. I know that's not the way to train him, but I needed to get his attention and break his frenzy. It did work. I then told him how bad he was. I came home, called A and then cried. We are going to get him private lessons with the owner of the kennel we go use. We've been told he works wonders with dog aggression. I hope he can work his magic on Boomer. I love my dog, but I can't deal with this behavior.