Aussie Training Help: Herding instincts leading to biting

I saw the warning signs and now the issue is more real than I’d like it to be.

I work construction and bring my 18 month old Australian Shepard to work with me everyday. For most of his life he lived off leash at my worksite about 50% of them time and tied up on a long lead outside the other 50% of the time. Very randomly throughout his life he has nipped at the back of my workers legs when they’re walking around the job sites. Unfortunately he never does it when I am around and I never have a chance to tell him not to do it. Recently it has become more aggressive and today he actually broke skin on the back of my hvac guys leg. He was tied up and the guy was just walking to his van and apparently Jack(my dog) rushed at him and bit his leg. Didn’t hold on or anything but it was not tempered enough as it did break skin(not deep but enough to bleed).

I’ve been reading other stories about herding breeds and how to curb this behavior and most involve “recreating” the tendencies. He doesn’t show these tendencies at any other time and is amazing at home, with friends/family, with other dogs etc. It seems to only happen when I am not around and when we are at the job site(most likely because it’s the only time he’s surrounded but lots of people he is not totally familiar with).

Anyway, looking for advice as the next time the person may be much less understanding and make this an issue that has drastic measures. I also hope to keep bringing him to work for the rest of his life so need to do something about this ASAP!