well guess what, i have a dominant male english mastiff, and with certain dogs he used to go completely ballistic until i figured out how to handle him. believe me, it was a very steep learning curve for me, as i'm a pet owner and this is the first time in 50 years i've ever owned a dominant, hard dog. and this dog is very hard.
what some posters here have already said: you've got to be on the ball and paying attention to this dog's body language at all times. the second the ears come forward and he starts to fixate on another dog, he gets one warning "NO" or "UH-UH" and if his ears don't go back down and he doesn't redirect his attention to me in 1.5 second flat, he gets a HARD correction on the prong. keep moving through the whole thing and turn away from the other dog at a 90 degree angle and just keep moving.
i use the big prongs. even though he's not as responsive to the heavy prongs as he might be with the little ones, he's a hugely powerful dog and i need the heavy prongs for him, just for safety's sake.
what is the history of the dog? was he acquired as a puppy? did he have a dominant personality? was he ever attacked by another dog? basically what i'm getting at is, is this fear or is it dominance?
i went through a lengthy period in my dog's rehab where i didn't permit him contact with other dogs in public, so i'm not buying it that you can't avoid them. even if you have to put him in the car and drive him to a deserted area at night to exercise him, do it.
i would not recommend using a dominant dog collar on a 200 lb. mastiff. from everything i've read, that could turn into a real nightmare. i think you probably need to go to an e-collar if you can't work with him on a prong. i have never used an e-collar, but when i was researching how to handle my own dog, i became convinced from my reading that an e-collar would be my next step if i couldn't redirect him with the prong.
the whole secret is training him to never even look at another dog. by the time he goes ballistic, it is too late, and you can't control him. i used to look at him, see that he was about to go completely nuts, and feel helpless not knowing what to do. that never happens any more, and we can walk right by other dogs and he ignores them, because he got his corrections before things had a chance to escalate.
i'm a 130 lb. woman and the dog is 150 lbs. of solid muscle, so it can be done, even with a giant. but if my dog is in the "red zone" with another dog, there is no way i would think of alpha rolling him. even a leash correction will cause him to redirect his aggression to me once he's that far gone.
best advice: get good professional help live and in person. this is nothing to fool around with. good luck!
working Mastiff