biting and nipping

Question
I have a 2yr old JRCT, rescured last nov, he is a great dog and the training is going well - but he has a tendency to growl at strangers, and if off the lead will chase and nip their ankles (which happened today!)I try to be aware of this but do not always spot the person in time. No amount of calling will make him stop - help

Answer
This happens a lot with rescue dogs, it's sad but they are nervous and protective because of how they were originally treated by humans.
You shouldn't let him off the lead -ever- if he is doing this. It could escalate and get worse and then you could get sued and he might really hurt someone.
Keep him on the leash and close to you, and practice with someone walking by.  
The SECOND he starts to growl you have to immediately correct the behavior by yanking on that leash and giving him a poke/squeeze to the back of his neck. It has to be almost instantaneous and you have to do it every single time it happens. Consistency is the key. Do it over and over and over. You are "being" his mother - when mother dogs correct their young, they don't yell.... they nip and nudge the pup to correct the behavior.  You can still say "NO" when you are training your dog but calling and yelling at him does nothing especially when he is already in the run/attack mode. Practice makes perfect.
Maybe you could go buy a 15 or 20 ft training leash and keep it on him while you are correcting this behavior. It will give him a little more "freedom" but give you the option of grabbing that leash before something happens.
Good luck