Infant Halloween costume ideas?

I haven't decided if I want to make his or buy one but I am also stuck on what to dress him up as. He's 9 months old and I asked my mom what I should dress him up as and she said the grumpy old troll on Dora the Explorer because when I get up to do stuff around the house he starts crying so she starts singing that song and he starts laughing and goes on his marry way. Anybody have any other ideas of whats unique and would also keep him warm?