Gracie Ruth is my
Alerter. She tells me about everything that is going on. She has "issues" when we are out, with other people, kids, dogs, and does a lot of barking at them too. In our home she is ok. So to me this says she has some security issues. She feels she needs to bark and tell them that she is one tough 4lb cookie and they'd better stay away.
I do a touch with my fingertips on the back of her neck and tell her Quit! I'll turn her towards me so that her focus is broken on who she's barking at and is focused on me. We'll continue on and if she starts barking again we'll stop and do it all again. I'll continue the touch a little firmer each time and the Quit command. Keeping my voice calm, firm and low. At first you may need to do this many times in a row until he gets the idea. Usually she'll stop after a few seconds. If she doesn't and still squirms around to see them I'll even hold her chin hairs a bit to break her focus and get it on me. She is 11 months old now and I rarely have to do it more then a few times anymore. I see it getting better as she gets older, and she sees that I am aware, in control and will protect her. Don't pick him up, don't comfort talk to him (that just tells him that he was right in being worried), you having a calm quite secure attitude is paramount in getting him to calm down too. They feel what you feel, if you are stressed because he's barking then he'll feel the stress and all the more think that there's a reason to be worried. If you can get him a bit worn out with play before going out it will help his energy level to be down some and he'll be more likely to focus on you quicker. If not, just be patient and consistent, it'll come.