The barking and hackles can also be from EXCITEMENT! All my dogs do it all the time even when they are playing with each other. LOVE the great job you've been doing socializing.
Using the collar/leash is a good idea, cause you are showing him you are in control and that the new person is fine. You may want to add TREATS to the mix, cause eating can be a calming thing to speed the meeting up. Particularly when you hand the treat to the 'stranger' and they feed your dog. They can also ask for a 'sit' and use the treat to reward.