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Well if your son took Draconic as a language he can talk to the dragon. He doesn't need any skill check to communicate with him but you can ask for a Diplomacy check if he tries to negotiate with the dragon (although it will be very unlikely to have any effect on the Dragon).
If you plan to run more adventures featuring Black Fang then this first interaction could be more interesting if there is some sort of dialog before the fight begins. Maybe the Dragon taunts the characters or tries to bargain with them - asking for their treasures in exchange for their lives, maybe?
That way, on the next adventure the players will be fighting Black Fang instead of a "nameless" black dragon.