My puffers usually do that after I disturb their tank after a water change, I'm assuming it's a sign of stress. They do like to "rest" in that position in hollows in the sand at night as well, though not as tightly curled.
How long have you had him, in what sized tank and what are the water parameters? Has there been a sudden change in salinity recently? What have you been feeding him? If he won't take any meaty foods like frozen bloodworms readily, he's likely very stressed and possibly sick.
