What does being put on trial docket mean?

It means that the next time he goes to court most likely, he'll be going to trial.

It means the case is set for trial on that date. Usually, a court's docket on a particular date will be comprised of a number of cases set for trial on that same date. During the week before that date, the court administrator will arrange the docket of cases in the order the court will discuss them. One or more will proceed to trial (in sequence), others will be be moved to a later docket date.

Trial docket means the judge has set the case for trial on a certain date and time, along with other cases. His numerical place on the trial docket is no guarantee that he will or will not go to trial on that date. It's a warning to be prepared to try the case that day. He should not be surprised if the case is "called for trial." He needs a lawyer.