An hour is a long time to expect a first grader to sit still, and impossible for a child that age who has ADHD.
Is it possible for him to participate on a tablet or phone so that he can walk around and move during the class. “Still” just isn’t a reasonable expectation for that long at that age.
Some strategies in this article might help with online schooling overall too:
ADDitude Community Moderator, Parenting ADHD Coach & Author, Mom to teen w/ ADHD, LDs, and autism