Thanks, good suggestions.
I don't think he's tall enough for Dumbledore.
Ice King or Petrikov might work. My wife is thinking about doing Marceline anyway, so they'd go well together.
Huang Zhong is also pretty cool, though that cosplay is pretty elaborate and would take a fair amount of work to put together well..
He's leaning toward Captain Okita from Space Battleship Yamato (aka Star Blazers in the US):
I think he should just do Santa all the time. Everybody loves Santa, who cares if it's March?