Please do NOT put a choke collar on a 5 month old puppy. Sounds like you need to take him to training classes. You need to do NILIF
(Nothing in Life is Free) with him and he may not have any Bite Inhibtion and you need to teach him that as well especially if he didn't learn it from his mom and littermates.

Bite Inhibtion -
http://www.paw-rescue.org/PAW/PETTIP...Inhibition.php

Nothing in Life is Free -
http://k9deb.com/nilif.htm