A valuable friendship does more

Keep regular and sincere discussions,do not go too much beyond your role. When it's time, both of you will know. If not, a valuable friendship does more! That's enough!

Just give him the impression that you regard him as a good mentor and sincere friend that you cherish very much! Becuase he is the type of person you want to become: his humble attitude on research, his sincere character, his generosity in helping people.