Ignore him and try to build a relationship with his kid. Most people who were exposed to religion and gone off the deep end is because of their Taavos. Their Taavos then tell him/her -Don’t worry about anything because there is no G-d. This is not possible to change with reasoning/talking with them. An extraordinary event has to occur for them to change.
On the other hand, the kid can be prevented from becoming this way. He can be exposed to positive things in Yiddishkeit and these can make a lasting impression. Don’t rely on the Yeshiva to do this, try to have this done during the rest of his time. Like invite the kid for Shabbos meals, etc.