It may be a side effect, but it also may be a dosage problem. The medication may be working partially, but not taking care of all the symptoms. He may not be feeling despondent, but he may still feel rotten and is taking out on everyone he meets or sees.

The pdoc may need notifying, although you seem to be moving away from helping in any way. Maybe it's best to be brutally blunt about his behavior and let him take care of it himself.