It’s probably best to tag one of the forum admins (@joshua or @Lien) so they can look into this for you. To help them, could you post a link to the course(s) your son is doing and say if he is using the website version of Memrise or the App?
Ah, the leaderboard icon on the bottom displays your overall points accumulated from all your courses, and only your mempals will be shown on that leaderboard.
To see individuals courses leaderboard, you will need to tap on the course title.