Annik, I think that's lovely, and I'm sure he appreciates the help! I would have to agree with you too when you say you don't force your own level of discipline but rather back his up. That's, in my opinion, the best way to go.
And Leaning, you want to say smaller kids are easier when you have this 3 year old that we watch daily... Whoo, boy! He's a mess, and his own mother says the same thing. It's amusing sometimes, but others, it's like WOOOO boy! You just want to pull your hair out. He's not my kid, though. He's not even technically my nephew, but if he does something he shouldn't, I do correct it.
Though, teenagers are sometimes difficult as well. I won't deny that. Can't even say if it's easier or harder to talk to teens than a toddler or such. Some teens will talk, and some don't talk at all to anyone.