Hey i also agree that your dog should always be kept on the lead untill he knows his boundries.
I dont know if you have eve herd of the dog whisper??? My new dog (poss with a bit of staff in him) was very protective of me, and everyone in our house inclding our other dog jake.
I maily used the cesars techniques and he is now a lot better (have had him 4months)any time on a walk you see his body language change i.e eras up, tail up body tense you need to destract him either by a side ways pull or a noise and carry on walking. Try to keep him at your side or slightly behind you and stay calm when he become tense with repatition he should calm down