A detective told my son while in juvenile custody that if he caught him on the street he would mess his life all up the jdc guard witnessed it

The focus here should be on the fact that your son is in a juvenile detention center and not on some comment by a Detective. If you raise your son and supervise him properly, so he is not hanging around criminals or enaging in criminal activity, then there is nothing to worry about regarding such a comment. Otherwise, if your son IS a criminal or hanging around with a criminal element, again there is no issue with the statement as your son, not the detective, will be "messin his life all up" and the law enforcement officer will be doing exactly what he is there to do, arrest and help convict your son, as he would deserve. Other than that, your remedy is to file a conplaint with internal affairs, at least to document  the statement should something improper happen in the future, but don't expect anything beyond that with the facts presented.