Duel enrollment usually means a course that is a college/high school course…not another high school course.
Some schools do allow online courses. But they have to be approved.
In my state, a student would have difficulty getting high school courses from another high school added to their resident high school transcript. It just would not happen.
But a college course in conjunction with the HS…to satisfy the HS graduation requirement would be.
Your son had a health issue. Did he have a health plan for educational purposes while he was recuperating? Does he have a 504or health plan now that he had started school again? If he has a plan…contact your case manager and discuss this with them.