He's had asthma attacks before. Kind of a pain, but a fact of life. Not even that scare, since he has a rescue inhaler. 

It's not here. It's probably downstairs, and if Theo weren't in the middle of making breakfast his uncanny Scott- sense would have already kicked in- but instead, he's downstairs, and doesn't notice until Scott has already tumbled down the stairs.

When he hits his head against the wall, Theo is rushing to his side, but he's already gone.