Given time and a lot more socialization, he will naturally learn to bark when confronted with a direct threat. Without a direct threat there is no reason for him to be barking. The last thing you want him to be doing is barking at strangers for no reason.
Be glad that he's very friendly. That's a good thing. The chances of him ever being put in a position of defense is slim to none. Having a dog that barks aggressively at everyone is a lawsuit waiting to happen.
Leave him be and see how he turns out as an adult. It sounds like he's a great dog.
Elaine and the herd