My son is playing this and I am the designated helper. He doesn't always read what someone tells him. Is there a way to get them to repeat it? Specifically Tom Nook... My son cannot sell or buy anything from Tom.... When we talk to him, he just says, "Have you finished your task..."

Is there any way for me to get him to tell me what task he is talking about?

He wants your son to talk and meet every single character in his town, which includes the two characters inside his town gate, all the characters in his museum, including the second floor, the two characters in the Able sisters shop, the character inside the town hall and all the current residents of the town. Also, after that task, Tom Nooks gives some letters and has your son deliver them to the corresponding characters, he also asks to plant some flowers around his shop. All these tasks must be done in order to be able to get on with playing the game.
I hope this helps.

Tappy