
hi,
sounds to me like he has learned that barking at things he is scared of, ie people, makes them go away! an, since it works, he will keep doin it.
What you need to do is twofold. Firstly, stop making the behaviour rewarding for him, so yes, making him sit down, whilst people pass is a good move. Secondly, u need to get him to associate people with GOOD things, so you need to get him to associate people with treats, either food or a toy.
The best way to do this, wich is quite hard, is to get strangers to give him a little treat, when u have got him sitting! Get them to walk past and just drop a treat beside him. If he is barking then NO FOOD, if he is quiet, then a treat appears.
You can then graduate on to getting ppl to pet him and give a treat eventually.
Good luck,
Em