And the first thing to do is establish to the dog that you're the leader, and you control them and the walk. Durango is behaving this way because he does not accept you as the leader and believes he is, which is why he is the one acting out on walks. He thinks because he believes he's the leader that he has to protect the group from any threats that HE perceives whether they really are or not.
Training by itself is not going to do the trick. He may listen some of the time or even most of the time, but in any instance where he feels that what he should be doing is necessary he isn't going to listen to you because he thinks that HE is the leader and the one to whom the protection responsibility falls.
Leader dogs get to do everything first. They go through doorways first, they walk in front of everyone else, they eat before anyone else, they go up and down stairs ahead of anyone that is with them, etc. Leaders also decide what is a threat and are first to make aggressive overtures to anyone or anything they see as a threat whether you do or not.
The first thing that's needed here is to reestablish the proper relationship between you, the other humans in the home and Durango. The good thing is that old dogs CAN learn new tricks. He'll be puzzled as hell when you first start establishing who is the boss, but mostly he'll likely be relieved... most dogs don't WANT to be leaders. Being the leader means having all the responsibility and making all the hard decisions, and like a child what child wants that? I think it helps a lot to figure out how to be the leader with a dog if you consider that it's not really so different than establishing leadership/parenting over the children. As we all know, good parenting means not just loving your kids but raising them... the parents make the important decisions, establish and reinforce boundaries, establish ways to getting through to the kids who the parent roles belong to, eeking out appropriate punishments for bad behavior, etc. If you're a parent, this should make a lot more sense to you than it did to me who has never been a parent nor even a quasi-parent as an older sibling. It did help me a whole lot to recall how my own parents did it since I was blessed with two really great parents and then transferring that into rearing a dog with how dogs think.
First comes learning though... learning how to be the dog's "parent" (leader) and how they accept or don't accept that role. Then comes the training... how to teach them what you want while out walking and what behavior is not acceptable and how to correct the bad behavior.
You also need the proper equipment to be better able to both control and correct the dog. There are various kinds of collars and leads for help in controlling and teaching the dog you'll need to learn about and you might find that you have to try a few different ones to find what works best for you and the dog. Having large breed aggressive prone dogs I've found that what works best for me is a basic prong collar that he only wears when on the leash. Basically, you need something that helps physically control the dog as well as a device that can be used to correct bad behavior. That said, you need to control the head... where the head goes the body follows. With a large breed dog that is pound for pound FAR stronger than a human what equipment you use is important. Halters do NOT control a dog physically unless they are small dogs and do NOT provide a way to give the dog corrections. Halters were designed to allow the DOG to lead in specific situations where they are the ones that must such as sniffer dogs and dogs that lead disabled people. They give FAR less physical control and aren't meant to give control but to give a necessary LACK of control.
Leerburg is a very excellent website for dog training and not only sells various equipment that is hard to find elsewhere shows you how to use it properly. They also have TONS of free information at the site though they have their own various training videos if you're interested in them. I would recommend reading there first with the whole understanding and establishing leadership with a dog that pretty much starts here...
http://leerburg.com/philosophy.htm
You CAN fix this issue but it's going to take time and a lot of consistent work. It also has to be something that everyone in the family is on the same page about. Luckily, being the only "family" with my dog I never had to deal with that and once making up my mind what formula to follow I didn't have to worry about anyone else sabotaging what I was doing or even disagreeing with either the philosophy or the methods. How you decide to go about fixing the problem means that everyone else in your family has to be in agreement about and share in the work.
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