Please feel free to address your concerns with the school Speech and Language specialist and program for the hard of hearing and deaf in your school district. If he is not yet in school, you can request a meeting from the early intervention team (some areas, this is covered by a Child Find program separate from the school district).
The best way to confirm comprehension is to ask the child to tell you about the day's lesson. For example, "Please tell me two things we talked about during Social Studies." Or "Please tell me what your homework assignment is for tonight."
Never, ever ask if the child heard you. Most school kids in the mainstream are not deaf. They hear. They miss parts of spoken language. The question is did they hear accurately?