Disclaimer: These characters (and Glee) aren't mine.
Eight years ago, his wife passed and he spent six months tanked up on Jack.
He awoke one morning to see Kurt sweeping up broken glass and he realised that he'd failed him; left him to heal and attend himself when a grown man couldn't.
He started to redeem himself that day, teaching Kurt about cars despite feeling like a wreck. He hasn't stopped since.
Eight years ago he made an appalling mistake, but now he's telling Kurt that he'll always love him, no matter what. The answering hug relates that it's never too late to become a good Dad.