It can be confusing and you'd want to follow up and explain to him in age appropriate way why not. For example, you could say that ministering is to be left to grown ups who have studied and proven themselves as Christians.
You may give him an example from the Gospel where Paul talks about how in the past his understanding had not been complete. He compares his understanding before finding Christ to one of a child and after that to someone more knowledgeable.
You can also let your son know that he can teach by example through his positive behavior.
He is not my son; he is the son of one of my co-workers and he likes to visit me when his mother brings him to work after school and this was something he was either complaining about to me or asking me for my opinion on or asking me for advice about. He made it appear like his parents were shy to preach to others about God. I stress though; it just appeared that way. Is your answer still the same or is there anything you would like to add in light of this extra information that I have given you?