If he's running around with just the harness and no leash attached then he's most likely getting worked up and coughing/hacking.
Mahina used to do that until maybe 4-5 months old. I havn't noticed her do it in a while and she'll run around like crazy.
I'm sure he LOVES to explore, all kinds of new smells!
ETA: If you can fit a finger underneath then maybe it's not too loose.