Does my 14 yr old mumble because he's not comfortable speaking

by Tanny
(Southport, NC, USA)

My son does this thing where he clears his throat each time I ask him to speak louder. He mumbles most of the time. But he definitely can speak more fluid and louder when describing something he likes or that he's interested in.

When meeting a stranger or even someone he knows he sounds very scripted (robotic) just saying hello. I can tell that he's not comfortable but I don't know what I need to do.