There are tons of videos on YouTube re: training dogs, specifically Heelers, and dealing with their specific issues. It's really hard to give advice without seeing the dog and watching how he reacts to certain things. Generally speaking, when he misbehaves (like when he nips) correct him sternly but calmly and redirect his attention to something else. Make him sit, for example. And when he does give him a little treat (doesn't have to be much...just a piece of his kibble or something like that).

I can't stress CONSISTENCY enough...in your commands, your body language, when/why/how you correct his behavior, etc. That's probably the hardest thing to do.

There's a book out there, very old school and probably out of favor with the times, but from a very talented dog trainer. The ideas and reasoning are very sound...I bought a copy back in 1989 (I think) when I needed to train an incorrigible black lab...turned out to be a fantastic dog.

https://www.abebooks.com/Koehler-Met...waAsfMEALw_wcB

It's an easy read and has some great information.