Now I'm confused ---
First you say he's 'going through a phase' in your original posting.
They you say it's "Nothing new" and has been occuring since he was 6 months old. Which is it????

:(
Basically 'a phase' doesn't last 2 years. :(
If he's had behavioral problems (indoors/ outside -- doesn't matter where) since he was six months old, then his training became noneffective when he hit the 'teenage years.' He gained control in your household and no one has ever let him know that he was 'out of place.'
Yelling, hitting, choking him out wouldn't work to keep him in line for long....only consistently reinforced formalized obedience training does the job.
You say he "tries to dominate everyone he meets" --- exactly how????? It is dangerous. If he hasn't bitten someone yet, he will probably soon, especially since you are saying that he's now "snapping at visitors without reason." Well he has a reason, regardless though it's unacceptable. :(
You have basically a ticking time bomb for something bad to happen and become another situation in the news blaming rotties and other "aggressive breeds" for dog attacks. Your insurance, your income, your dog's life, and someone else's life is at stake if you don't get him under control. It will only grow worse on its own.
Contact your nearest Rottie club to see if there are some training programs and specialists that might be able to help you out. The next step possibly would be to give him over to some one who is able/willing to get him trained.
I think others on here would agree that there is no "cure" except expert handling.
If you cannot or will not do this, expect the worst someday to happen. There are enough incidents of 2-4 year old intact male Rotties to not anticipate an attack to happen. All it takes is someone the dog percieves as "weaker" to do something he does not like for a serious bite/attack to occur.
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On the other side of the fence that this has not been gradually growing from 'teenage' years ------
If chasing shadows and being aggressive has occured recently and is different from his normal personality, then something in the brain/eyesight is very likely. He needs to be checked out by a vet immediately.
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I am not writing this for you to be insulted or to defend against it. Seriously, I'm not angry or mad at you. I'm not wishing to start a flaming argument :rolleyes: :( Please just read and consider. If you don't like what I say, just ignore it. It's ok... It's only my opinion about what you've said.
He might be the sweetest dog around when he's behaving. But he's not reliable. He hurts visitors. He's now 'snapping' at them. "He tries to dominate everyone." He can get out of a muzzle. Your house has been chewed apart over the years. Rotties are a very heavy strong dog breed to not take this seriously. It doesn't matter what I think about him. It's what you've written about your dog.
It all comes down to:
Are you willing for someone in your family, friends, or even a stranger to get seriously injured or maimed by your dog?
good luck and you have my best wishes,
toodles