I'm pleased to learn that your son's hearing (and speaking) is helped by the BAHA device but I believe that your question is beyond the scope of our members and best answered by your pediatrician. However, if any of our members have had a similar experience, I'm sure they'll respond. I would assume that at some point in the future, testing by a speech-language pathologist would be helpful for ascertaining your sons speech-related needs. I would guess that being verbal around him as much as possible would be a good idea, but I have no real expertise on this issue.
I trust that the BAHA device will continue to help your son and that, with your assistance, he'll develop a very satisfactory speaking ability as he grows.