Thank you for your query.
Tongue can get black coloured due to:
1. Intake of oral iron preparations.
2. Mouth breathing
You mentioned that your son had an ear infection
in end December for which he was on medication. The ear infection is also associated with a bad throat and blocked nose for which the child must be breathing from his mouth. This may have caused the blackish discolouration.
I hope I have answered your query to your satisfaction.
Wishing your son all the best.