In figure mode, you cant change the shape of the object.
If his legs are modelled flat, they'll stay flat until you move the bones in sequence or scene mode.
Set a nice default stance/pose in sequence mode, and set the sequence to 2 frames long. (setting a sequence to 1 frame used to cause some problems, not sure if it still does, i always use 2 frames, with necessary bones keyed in frame one.)(make sure green key on when moving bones)
Name sequence something like 'crabby stand', or 'crabby default' and just add it to crabby when you get to scene mode.
Sequences can be used as 'poses'.. rather than a moving sequence. Really handy. Export sequence to a folder like 'crabbySEQ', ready to use any time in the future.