no, he is not a MA state trooper, otherwise, he would have known;
1st- Operating unmarked police vehicles on the highways of the commonwealth of Massachusetts is legal. There are no Massachusetts laws that prevent unmarked police vehicles from making traffic stops on a highway and issuing you a citation.
2nd- there is no "law of entrapment" I think what you were referring to however, was that driving an unmarked police car creates an environment for entrapment, which is also not true. Entrapment is enticing you to do something you would not have normally done. Simply driving an unmarked police car is not enticing you to commit a crime.