Well if i compare this to my dog, its sounds a lot like her in some ways..
So it could be fearfulness/ being unsure of the outcome of the people/dogs.
Some dogs will take treats and than bark again. It's happened to me before, kennel aggressive dogs will take treats from me and stop barking during that moment, and begin again after the treats are gone-it defiantly wasn't because they wanted another treat.
If you watch his body language and see if he is doing any calming signals..
And those are most extreme and exaggerated body languages.. But read your dogs energy and his body language when it see's a person or dog.
Find if your dog is doing calming signals and give calming signals to your dog such as yawning, turning to the side of the "Scary thing", looking away, sighing, walking away etc.
If your dog is fearful aggressive, than its best to go slow, and at their pace for counter conditioning.
For the fear aggression i REALLY recommend this booklet by Patricia McConnel, it has really helped me with my fearful aggressive dog to strangers!!
(even more than my private trainer in some ways honestly!)http://www.amazon.com/Cautious-Canine-How-Conquer-Their-Fears/dp/1891767003/ref=pd_bxgy_b_text_z
This book is really good for new dog owners, http://www.amazon.com/The-Other-End-Leash-Around/dp/034544678X/ref=sr_1_sc_2?ie=UTF8&qid=1386555824&sr=8-2-spell&keywords=Patrrica+mcconnel
And this last book is also really good for any dog owner: http://www.amazon.com/Leader-Pack-have-Your-Love/dp/189176702X/ref=sr_1_sc_1?ie=UTF8&qid=1386555824&sr=8-1-spell&keywords=Patrrica+mcconnel
You have to protect your dog from circumstances where they will be so scared where they will hurt someone, also protecting other people. If you have to buy a muzzle, than do so. But start working now and go at the pace of your dog. When you rush your dog, you will be going over his threshold and in return will put you back.
Be sure to also give your dog lots of exercise and mental stimulation. I know i've read and seen myself with Zelda, that if they do not have that exercise they have more pent up energy they can put to their fear and aggression. When I get Zelda from her crate after work, after she has been in there for hours. She is more reactive to new people than after a long walk. Also mental stimulation helps build up your relationship with your dog.
This book i recently purchased and have only used a few things in it so far: but it looks like there are a lot of fun ideas to do!
Brain Games for Dogs: Fun Ways to Build a Strong Bond with Your Dog and Provide It with Vital Mental Stimulation
A lot of it is about management and takes time to get to know your dog, its threshold, and body language. But there are a lot of great resources out there.