Humping Puppy

Hi Guys,
 
I have question about humping.  My pup is about 5 months now and he started humping a couple days ago.  Actually, I noticed this behavior when a friend dropped off his year old male dachshund (neutered) off at our place for us to dog sit him for a couple days.  This is when my puppy started this humping thing.  At first, I thought maybe he was just mounting the dog to let him know that he's alpha, but this morning I noticed that he actually was erect when I pulled him off the doxie.  And then it shrink instantly.  Is he going through puberty already??  What should I do?  Is this a behavior that needs correction or should I let him be?  I should also mention that my pup isn't neutered.