| This is a problem I've had a lot with sheepdogs, because the herding instinct is sooooo powerful in them. They nip and grip, which represents an escalation of the herding technique. They'll start off with basic steering; if the sheep doesn't comply, they'll progress to a nip to persuade it and if that doesn't work they'll up it to a grip. The problem is that sheepdogs will do exactly the same thing with children that are running around, and then it can become a big problem.
How to deal with it:
Firstly, you need to ensure that he cannot do this. That means keeping him on the leash anywhere people are likely to be running around.
Then you need to desensitize him to runners. This means controlled, on-leash exposure to runners combined with training him to ignore them. I could explain in detail how to do this but frankly you should really work with a trainer rather than get this sort of advice online. It's not difficult - it involves gradually getting him closer to his triggers, recognising exactly when he goes into "too much interest" mode, and snapping his attention straight back to you on command. Clicker training is excellent for this sort of issue.
All this will need to be done on a leash until you are 100% sure that either the behaviour is broken or you are able to stop him in his tracks if it starts. But do get a trainer - it would probably only take a couple of sessions for a trainer to show you exactly what to do, when, and how.
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