What should I have done to help train my dog in this instance?

I completely agree with Kaper. Hire a professional. It's better to correct this now rather than let it get worse. Do you want to fear what your dog could do to someone for the rest of his years? Do you want a potential lawsuit? A professional trainer is a lot cheaper than medical bills and pain and suffering that you could get sued over, even if it was only a nip. You will enjoy his company more too.