Are you a member of backyardherds? It might be a good idea to post on there, it is a sistersite to BYC. He could be hoarse from crying too much (and I know bottle babies can be LOUD!), or he could not be feeling well. Bargain asked some great questions for more detail.
I recently lost a goat kid, who was nearly three months old. One of the first things that stood out to me before she passed (despite treatment for bloat, what she was displaying signs of) was how quiet she was. This kid normally would scream up a storm until I gave her my direct attention. So when she didn't even mumble when I went into the barn, I knew something was very wrong.