I recently joined a company as a network engineer. I was trained about systems in the site by the team leader. 

He didn't train me properly. He kept secrets which he should not have, like log-ins and some customizations of the system. This is not a smart way, because once I have access to one system I can explore from there, and I have. My boss have given me all access except a firewall and I said that to the team leader. 

He became the team lead because everyone else left. Technically and interpersonal skill wise he is not that competent. Definitely I can say I'm better than he. I'm messier than he is. One of my colleagues had the same feedback about this person and he told my colleague he is afraid of getting demoted.

So, my problem is how can I get this person to train me properly and disclose everything he knows to me while talking him out of his fears?

"I'm better than him" is not my opinion but my manager's. Me and manager had a discussion about how we are going to improve network security, performance and control. I am going to work at this particular site because this team leader lacks of technical competence. As it seems this person have lost trust with management also.So I am instructed to do particular things and I need this person to help and I always will be in his help.