Well he said bite and not have sex. I find it funny and charming the way he defended his grandson. I interpreted it like giving them blessing for marriage and also saying to parents leave the "kids" alone.

Probably Brandon mom was trying to control the situation and put words in grandpa's ear. Who knows for how long he was listening her nonsense and words like"girl from Russia"..

So when they arrived and he saw them probably he thought, Betty you can talk all you want but this "girl" is beautiful so let him make mistake, I would do it if I was in his place. He used word "young" but considering his age, she's young and she is girl (not lady because she danced in the club, not "woman" yet, didn't use word "kid" .. He's so far the star of the show !