The teacher is right. Those are first grade skills. I wouldn’t be too concerned that he hasn’t grasped the concept yet. It’s at least a year ahead of him developmentally. Sounds like your speech therapist is a little ahead of the game, and when his brain is developmentally ready, it will click.
But if you do want to challenge your little one now, you can use any group of objects and take turns playing “Which one is bigger?” “Who has more?” and “Who’s next in line?” by lining up toys or animal crackers or anything else. ADHD learners tend to be hands-on learners and have trouble imagining abstract concepts. Plus, any time you can turn learning into a game (and any time he gets a turn to try to challenge you) you’ll have him focused and interested.