Please advise about barking puppy

I have a 5 and a half month mini Schnauzer who is generally quiet and doesn't bark. He has started to develop a habit of barking and growling at any visitors that come to the house. I try to distract him which works for a few seconds, but then he starts up again. I know there is a method to train him to be quiet on command - please can someone explain how this is done.

When we are out for a walk he walks quite well on the lead, except for when he sees people or other dogs. Then he will lunge towards them and bark - it can sound quite aggressive ( he made a little boy cry the other day which I felt awful about. ). I try to distract him with squeaky toy and also pulling him away, but he is very determined and focussed on his 'target' and doesn't take much notice of my attempts to distract him. If the person comes up to us and to say hello he stops barking and usually interacts well. Also when he is off the lead he behaves in a friendly way towards dogs and people and doesn't bark at all - wants to play! I'm a little concerned about the behaviour on lead - I don't want this to develop. Any advice would be appreciated. Thanks.