Sadly reveals Shatner's character as an insecure person (and asshole) to go off like that on a 16-year-old boy.
As described it would be obvious Wil was nervous. A confident, secure person would have tried to put him at ease by complimenting him and making him feel good. All of us who lived through that age knows what it is like to be a teenager.
Instead he went on the attack trying to cut him down. Was he feeling bad about being dethroned by the TNG crew? Was his toupee slipping? Or was he just realizing what a piece of shit he was turning ST V into? Only the Shat knows for sure.