Problem is, I think, KDB is a huge introvert. Just watch the first couple of minutes of this, it breaks my heart:
So for him to be quiet and sensible and stay at home...it's no trouble for him, it's what he likes best.
So although his behaviour is probably a great example for Jack, I suspect Jack needs some guidance from someone who actually understands what it's like to have that constant drive to socialise and that little spark of self-sabotage that seems to always be burning in the back of his mind.
Do City have any reformed party animals? I'd suggest Kyle but I'm not so sure he's reformed.