
Righto! First things first, I regret to say, you saw an idiot :-( Not a poor reflection on you by any means, there are plenty of these people around but they are outdated and incorrect. No dog can be 'dominant' over us or vice versa - they are a different species, and don't follow the 'rules' of wolf heirarchy (and incidentally, neither do wolves ;-) ). Also any behaviourist worth their salt would have advised you NOT to neuter a nervous dog without putting a lot of work in, for precisely the reasons you've experienced. I'm so sorry you and your boy have gone through this :-(
That said, let's see. I don't as a rule advise for aggression specifically over the net as I can't observe body language etc but, a lot of what you've described is pretty standard for a very stressed, reactive dog so I can give some suggestions for general handling that won't put either of you in an unsafe position. Your first goal is to reduce those stress levels. Now this is going to sound a bit odd and counter-intuitive, but bear with me. I think you need to drop his exercise levels a LOT. 3-4 hours a day is a lot for any dog but for one that is confronting triggers on what sounds like an almost continual basis while out, it's an intensely stressful amount to cope with. Think about people in high-flying, high pressure jobs - stock market type stuff - stress-related illness, mental difficulties such as depression etc have a tendency to go hand-in-hand with jobs like that and it's very similar for dogs. With him it sounds like it's manifesting as a dog that's trying desperately to clear any and all threats before they can damage him - pretty standard fare for fear aggression. Best defense is a good offense, as they say: get in there first. This is not a dog that's trying to be the boss - this is a dog that is panicking because he thinks everything is a threat and he doesn't know what to try and scare away first.
What you need to understand before I continue is that stress stacks and sticks. That is, every time he is exposed to a trigger (be it person, dog, whatever, anything he's worried about), his stress levels will increase. Each increase goes on top of the last, so they stack up. Each increase then hangs around for a few days at least - they stick. So, if he's seeing, say, 3 triggers one day, he's got 3 points on his stress meter; that might drop by 1 point the next but if he then sees another 3 triggers, he now has 5 points. Drop by 1 again but see another 3 the following day, and now he has 7 - so you see how it creeps up. Stress inhibits learning so when he's extremely stressed on a constant basis, you simply cannot make any progress with him. And of course the higher it's all stacked up, the longer it takes overall to drop down to normal levels.
Ok, so. Step one for me with that in mind would be to give him a complete break - a week, probably two from what you've described. This should be either not leaving the house at all or, if you have access to somewhere suitable, a deserted area or somewhere you can stay far enough away from his triggers that he won't be upset by them (think open fields, countryside, that sort of thing). This should give his stress levels time to drop down to a sensible level, and allow you to begin work with a fresh start. Do training with him at home, play, interactive toys, whatever you need to to keep him occupied but don't feel guilty - his walks at the moment sound like they are not enjoyable for either of you so he will enjoy the break!
Now you can start walks again after the break but, as I said before - much, much less than he's getting now. Remember, every exposure to a trigger will affect him for a few days at least so you need to minimise exposure. Often this means one or two short walks a day, longer if you are walking in quiet areas. If you do see a trigger, try your best to keep him far enough away that he's not fretting about it - this is your working distance. Here you can progress him with techniques such as Look At That or BAT (I'll link to vids in a moment). The key is to always keep him sub-threshold (further away than he starts to worry). At that distance, he'll be able to focus on you, give you behaviours to reward (such as moving away from the trigger, sniffing the ground, looking at you, or even just calmly observing said trigger without doing anything).
Right, that's a start; can I ask at the moment, how you are handling him when he does kick off at something? And also, what he's being fed on?
And one more question - have you tried any calming remedies/supplements for him?