sudden aggression!!

Maybe your bf startled him...?  Not that its an excuse but you said it was night, so maybe he didnt recognize who was picking him up... can you normally pick him up without problem?  Sorry Im not more help.

First, you don't train a dog to walk unleashed by just letting him off leash and hoping for the best. 
Second, I think Amy is correct that your BF might have startled him.(And was he, perhaps, angry at the dog for running off?  That might have frightened the dog, too.). 

Sorry to be a wet blanket, but a terrier off lead is a bad idea.  Not only will you encounter issues like the ones described, but you may very well end up with a dead dog.  A split second look at a varmit of some type across the street and off your dog will go.  Car coming, yikes.  So, terriers should always be on lead unless in a secure area.  There is no sense in my opinon of taking the chance of your dog getting killed or seriously injured because they follow the instinct that is bred into them.  As to the aggression, terriers are tough dogs that were bred to go down a hole people didnt want to put their hand into.  In that hole, they encounter an animal fighting for its life, so a growl, dentition, or even a nip is not really an unexpected outcome.  You need to work on obedience and maintainance (not letting the situation occur).  It is just part of owning a terrier.  Did that once, wont ever do it again.  I prefer a dog bred to want to follow my agenda, not theirs.[]