Answer:
add “However,” before “Greg”
Explanation:
The word 'However' is used to contradict something that has been said earlier.
The last sentence ''Greg had his mind on other things'' should be written as ''However, Greg had his mind on other things''.
Here, the sentence ''Greg had his mind on other things'' contradicts the sentence ''Greg’s father sat endlessly lecturing to him on Greg’s need to improve his grades''.
