I picked up on a very good point in 2parent's post:
Perhaps your son could start by picking one obvious area to be more verbal, and then as that became easy, add another. When my pitcher son was a senior in HS and team captain, he helped the starting catcher to "come out of his shell" in pretty much that way. Varsity starting catcher was only a soph and very shy and quiet, but a great athlete and very good catcher. My son started by saying, "Okay, John, remember to call out how many outs, nice and loud, before each batter." John wasn't too shy to do that. After another practice or two, my son suggested another thing for him to take charge of, and gradually John became - well, not a highly vocal leader, but more comfortable with some of it at least. (The coach had given my son permission as captain to coach the other players.)