My 22-year-old son was charged with misdemeanor possession of marijuana. Should he have an attorney represent him at bench trial or do it on his own?

As a first-time offender, he entered a diversion program for one year. He completed everything required, passed two drug screens, but failed a third one. He now has to face the charge. The judge declined his request for a public defender because it was his choice to stop working and return to finish his college degree. Does he need an attorney to represent him to avoid any jail time, or could he do it himself and expect probation to be extended? What are the risks of not hiring an attorney?