Arent sure what to suggest love apart from seperating him from the others and just keep an eye on him. A warm bath does normally make them a little more active tho. Is there any chance you can post a pic of him? You say he's sort of half out of his shell - do you know if he's still attached to his shell inside?
R.I.P. Daddy 07/07/09 - Love and miss you more each day xxxx