He could be arrested for DUI (if Im correct), but not suspicion of possession... like cheato said, they can search his car under suspicion but cant arrest him on it...
unless they smelled the weed in the car and could tell he was high.. then they could arrest him, but the charge wouldnt be suspicion of possession
but like teagz said, he probably has some on him 9 times outta 10 lol