I have a better one. Recently, I've had problems where Khem Val would occasionally just stand around unless I manually order him to attack (and he's not set to stand-by either). As my sorcerer is quite squishy, not having a companion doing their share of the work can make class quests somewhat frustrating when this bug happens.
I've also had issues on my bounty hunter where companions just stop being automatically resummoned when leaving a vehicle.
Had you attacked the mobs at all when he was just standing there? I don't know if this applies to your case, but when me and my friend would quest together with our companions out his companion would often times not attack while doing more difficult quests. We figured out that because he was spending his time healing me instead of attacking, his companion would just stand there since he was considered to be not in combat (or something). The moment he would hit something or get hit himself his companion would take off and start fighting.