I would suggest getting a behaviorist to work with. I think there are probably very subtle things happening with Nelson's body language that can help cue you to when he's feeling nippy and how to respond to it in a calm, structured way. Those are things that are best done in person, IMO.
I think a muzzle is an excellent idea in the meantime, but I would strongly caution against using one in off-leash or exercise situations. Dogs simply cannot pant well enough with one on for extended periods of time. Even coursing dogs don't wear their muzzles after a run so they can fully cool down. I would use a muzzle for leash walks and in the house if you've got a stranger there, but if you're concerned that he may bite someone out on an off-leash run, then my advice is to discontinue the off-leash runs.
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