Dog Training: Nothing in Life is Free : The Humane Society of the United States
Nilif is a good option to put some real strength in his commands at this age.
You say he's a working dog? I wonder if you mean he's working lines or you actually intend on working him in some respect. Either way, now is a good time to start working his mind. There are several posts on here you can find with different activities that exercise the mind just as much or more than the body(try searching indoors games/training).
Apsel was strong willed at this age(12-16), and I noted he was very adept with his nose, so I because scenting games with his kibble/treats. Small games first, and eventually he was working for a good 15-30 minutes finding his meals bite by bite. It helped a lot to burn his energy and reinforce that nilif approach.
As far as the nipping, there are many things that could be occurring in terms of why the pup is not catching on quickly. One, pups are very fond of putting their mouths on everything, so nipping will be difficult to break, but definitely needed. A few questions may help for further advice though. Are you the only one who issues corrections for nipping? How long and strict are the ignore sessions afterwards? Have you tried completely leaving him in another when he's too excited and nippy?
Apsel was a nippy little **** at one point. I started working the same way you mentioned, but didn't progress. One day he accidentally grabbed my nose and sliced it wide open. I just knew the next mistake would be someone's eye. So from that day on, I had 0% tolerance. First instance of him trying to nip/lunge/etc. play was over, and he was in time out.