Please take a moment to consider that your dog's unpredictable behavior, attacking young dogs or puppies can have a long lasting negative impact on that young dog, it is traumatizing for the dog (not just the owner) that is attacked ie: can cause them to become fearful/untrusting of other dogs for life.
I suspect your dog is not as 'fine' (comfortable) with other dogs as you think he may be, he may tolerate them, even interact with them, but he has a limit as to how long he can tolerate their behavior and it is likely only a matter of time before he does indeed physically injure another dog/pup and/or you when you are trying to pull him off the other dog.
His behavior has escalated over time, at first he growled, - then he began to chase, (both of these behaviors are used to create 'space'/distance) and now he has escalated to being physically aggressive - because his 'warning' signals that he is not comfortable in those situations, have gone unheeded so he feels the need to get 'louder'/ more forceful in his request to be left alone.
It is not uncommon for dogs to not 'like' all other dogs, (we don't necessarily like everyone we meet) or for adult dogs to be intolerant of the rude/frantic behavior of younger dogs. If that is the case it is best to avoid situations where they may have to 'deal/cope' with socially inappropriate young dogs. (Realistically, if a dog is unpredictable and is known to attack (out of the blue) he does need to be under control/on leash in an effort to prevent him from practicing the behavior - doing just that. Not just for his sake, but for the sake of the other dog.)
If you are not familiar with how to read dog body language and can recognize the signals they give that say they are uncomfortable with the situation they are in, it would helpful to learn how dogs 'speak' to other dogs and to us. So that he can be removed from the situation before he feels the need to escalate.
Guide to Stress Signals in Dogs - Whole Dog Journal (whole-dog-journal.com)
The book: On Talking Terms with Dogs by Turid Rugaas, is a good introduction to dog body language as well.
I suspect your dog is not as 'fine' (comfortable) with other dogs as you think he may be, he may tolerate them, even interact with them, but he has a limit as to how long he can tolerate their behavior and it is likely only a matter of time before he does indeed physically injure another dog/pup and/or you when you are trying to pull him off the other dog.
His behavior has escalated over time, at first he growled, - then he began to chase, (both of these behaviors are used to create 'space'/distance) and now he has escalated to being physically aggressive - because his 'warning' signals that he is not comfortable in those situations, have gone unheeded so he feels the need to get 'louder'/ more forceful in his request to be left alone.
It is not uncommon for dogs to not 'like' all other dogs, (we don't necessarily like everyone we meet) or for adult dogs to be intolerant of the rude/frantic behavior of younger dogs. If that is the case it is best to avoid situations where they may have to 'deal/cope' with socially inappropriate young dogs. (Realistically, if a dog is unpredictable and is known to attack (out of the blue) he does need to be under control/on leash in an effort to prevent him from practicing the behavior - doing just that. Not just for his sake, but for the sake of the other dog.)
If you are not familiar with how to read dog body language and can recognize the signals they give that say they are uncomfortable with the situation they are in, it would helpful to learn how dogs 'speak' to other dogs and to us. So that he can be removed from the situation before he feels the need to escalate.
Guide to Stress Signals in Dogs - Whole Dog Journal (whole-dog-journal.com)
The book: On Talking Terms with Dogs by Turid Rugaas, is a good introduction to dog body language as well.
