My name is ***** ***** I’ve been involved professionally with dogs in the health and behavioral fields for over 18 years. It will be my pleasure to work with you today.
Let me explain a little about how dogs think. To you, another dog grabbed him and was hurting him. To him, you pulled him up by the leash and thus he associates the attack with you. In addition, as his owner and boss (as he likely used to view you) it was your job to protect him from something like that happening. He likely feels he can not trust you and may even feel you caused his pain. Dogs can fail to make the connection with the other dog.
It will take you some time but you can regain his trust. You do have to be sure this type of incident does not occur again. You should always carry a hook ended umbrella that you can open if necessary to scare away another dog. It can also be used to put between another dog and your own. The hooked end can be used to grab another dog's collar and pull them off your dog. Another item is pepper spray. You can use that to get another dog to leave and often a larger will let go of a smaller dog when pepper spray hits them. It will likely get your dog as well but is better than being torn apart by the other dog. Your dog needs to know that you will not let this happen again. You need to be viligent on waks to ensure no dog sneaks up on you especially given that many dogs do sneak up behind and attack.
Over time your dog should see you being watchful and notice that you no longer let other dogs close to him and start to trust you again. You can help foster this trust quicker with obedience training. It sounds weird but as the dog learns and obeys commands, he'll gather more self confidence around you and learn to trust your behavior again. The following site is helpful in helping owners train their dog. Be sure and click on the link to the page on obedience at the bottom. and links on subsequent pages leading to detailed instructions.
This will also help his behavior if he decides he needs to be aggressive to other dogs before they can attack him. This often happens after an attack. A dog will go on the offensive and start acting aggressive toward other dogs before they can attack him to scare them away. Now if he does associate the pain with you rather than another dog, he should not be aggressive around other dogs.
It will take him time to regain his trust around you, but if you are consistent in your interactions with him and start working on training and protect him in the manner I described, I'd expect him to show improvement within a couple of weeks.
I hope this information is helpful to you. If you would like any additional information or have more questions please don’t hesitate to press the reply to expert or continue conversation button so I can address any issues you still have . If you do find this helpful, please take this opportunity to rate my answer positively so I am compensated for my time.