He (the 14 year old) will be watching your reaction to this closely.
He is old enough and smart enough to read between the lines and judge character for himself. His mother's and yours.
Your part is to act in character, WITH character, and let baseless accusations fall from you as water from a duck. When you are consistent and centered, it will give him a valuable parting lesson, as he gets torn from "his" home and the only "father" who has given him the time of day.
Show him that some messages are much truer than others, and don't need to be said with words.
Sending encouragement,
CLMI