It's a good possibility that he is nipping because of the breeds in him. Both BC's and Heelers are herding breeds and because of that they are prone to more things like nipping and barking at moving things. Heelers especially like to nip...hence the name heeler, they were breed to nip at the heels of the livestock that they herd, while BC's use all kinds of traits nipping and barking included to get their job done.

In order to give you some advice though I know that I would need a little bit more info on what you do when he nips and also what is happening around him when he got nippy.