Aggression

Question
I have recently rescued a boxer from a shelter.  He is about one years old and 55lbs.   He has been exhibiting some aggressive behavior.   While walking him he will have a fit. He will turn around on the leash and jump up to try to nip at me. He has gotten my arms and knees.  He tends to do this when he doesn't get what he wants.  He does not break the skin but leaves bruises.  After his fit is over and he calms down he becomes very docile. He can be this sweet loving puppy but I don't know how to prevent and/or pull him out of a fit.

Answer
Something might have happened to him while on a leash walking, some kind of bad history.

If he is fine all other times, then do not walk him.

I am hoping you have a fenced in back yard, and if so, try leash training in the back yard in a controlled environment. Walk him on his leash in the back yard using single word commands. This training can be for 10 minutes and increase until he learns.