We may not take our words seriously - but others do, and something said thoughtlessly or in jest can do just as much damage as words that are deliberately meant to hurt. No wonder the Bible compares our tongues to a spark that sets a whole forest on fire (see James 3:5).
The problem, however, is far deeper than what we say - because what we say is only a reflection of what is within our hearts and minds. A person may try to guard his or her tongue - but if their heart is filled with anger or jealousy or pride, it will eventually come out in their speech. We may not like to admit this but it's true. Jesus said, "For out of the heart come ... false testimony (and) slander" (Matthew 15:19).
This is why our greatest need is to have our hearts touched by Christ, for only He can take away the things that are wrong and replace them with His love and peace and joy. And this is what I pray will happen to your son-in-law. When God touches our hearts, we won't feel the need to put down other people or try to draw attention to ourselves.
But pray too about your own attitude. You son-in-law's words may hurt but ask God to help you forgive him, and to respond with kindness. The Bible says, "Reckless words pierce like a sword, but the tongue of the wise brings healing" (Proverbs 12:18).