Yes, he absolutely can be trained, at any age.
It sounds to me though, that you need more than obedience training. If he is sensitive to strangers, or feaful, or "triggered" by particular stimuli, you'd be wise to look into things like desensetization and classical conditioning. When you do obedience training, you are working from the dog's intellectual perspective - this is "operant conditioning", and it only works when the dog is "under threshold", emotionally stable enough to respond in conditioned ways he has been trained to respond in. If the dog is "over threshold", meaning in a very aroused/excited/scared/agressive emotional state, he is not necessarily able to respond as his training dictates. This is where classical conditioning comes in - it reconditions the dog's underlying emotional responses.
For instance, we start with a dog who has some kind of a strong negative emotional response to men. The purpose of classical conditioning work in this case would be to show the dog that men are good and worthy of a positive emotional response. It works exactly as with Pavlov's dogs: we expose the dog to the particular stimulus in very small doses and very carefully, keeping him under threshold, and at the same time try our best to make him happy - usually with food. Eventually the dog comes to associate men with food, and stops reacting negatively. This is called "counterconditioning".
A word of warning from personal bitter experience: I would not, under any circumstances, expose a dog with any kind of oversensitivity issues, be it fear or agression, to any kind of negative training methods: no choke chains, or shock collars, or newspapers, or yelling and screaming, or any punishment whatsoever other than depriving the dog of attention or specific rewards that he has to "earn" by appropriate behavior when training in "operant" rather than "classical" mode.
So please choose your trainer very carefully, because a trainer who is not familiar with the above methods, scientifically and experimentally proven to work best for this sort of thing, can very, very easily make the situation much worse, and cause the dog significant psychological damage.
A dog who simply pulls because he is excited to go for a walk can be trained in any number of ways and do well (though I still prefer almost exclusively positive methods - I trained Myst on a flat collar, and the greatest "punishment" she ever got was either not getting her treat, or a particular "oink"-type sound of disapproval). A dog who is really having even a hint of agression/fear issues can very quickly go from an iffy temperament to a potential serious biter when strong negative methods are applied.
Especially if you have a sensitive and loving dog, don't get into confrontations with him, maintain his trust at all costs, protect him from what he fears, and teach him not to be afraid gently, don't fall into the "alpha-roll" and "dominance" nonesense - all that will do is damage your dog and your relationship with him.
So - just be aware. I've gone through all of that. Myst actually bit someone when she was 7 - after what's called "traditional trainer" - that is a trainer who employs negative reinforcement/punishment/dominance theory - had spent some time with us teaching me how to use prong collars and alpha roll my dog. That taught me a lesson I never forgot: if something doesn't feel right, don't do it, and you can't lose by keeping your dog happy, while you lose a lot by making her miserable...
Off my soap-box now - there is years of work, thought, learning and listening behind what I'm saying here. I *so* feel for people who have dogs with this kind of issues...
And, btw, the first thing you probably need to do is work on his reactions to the vet - you shouldn't have to fight with him for his shots, that's really damaging.
Another note is that a muzzle is actually not a bad thing - it diffuses a lot of tension in both the dog and the owner often enough. But if you need to use it, the dog can be very, very easily habituated to it, and just take it as another leash - no big deal.
I can't seem to train my mastiff. He is great around my son,wife and me. But anyone else he gets very protective, almost aggressive. When he was young I took him place to mingle with other animals and people. He did great. Then one time at the vets (a male vet which he has never seen tried to pet him and he snipped at him,well he wanted him muzzled and Dozer did not like that. Stressed him out so bad. Well we don't use that male vet, always a female and he is better. Not great, it takes 4 people including myself to hold him down for shots. But no muzzle. I hate those things. Anyway since then he doesn't like anyone. I think that incident caused his fear of new people. He is a loving dog and very good with my 4 year old son. My question is. Can he still be trained at 6 years old,or is he set in his ways? I can't even walk him without him pulling me down the road lol. Any info or help is very appreciated.