They had his passport, so it would have been difficult for him to leave and risking arrest by either the Egyptian or Syrian police.
There's something odd about this story because neither a passport nor money should have kept him there, unless there was literally some kind of watch for him, and/or the cash situation was really that extremely dire. That said, he was able to survive for 4 years so money had to be coming from somewhere.
I suggest that he was maybe being paid a tiny amount, and that he felt it'd be better to 'stick it out' as a nearly worthless cog, than to take a risky path home to what might be nothing anyhow.