Tough issue. I can see it both ways.
1) He took a job knowing what the requirements were. I worked with a Orthodox Jew who was fired for similar reasons (he wouldn't work during the Sabbath and after a year things blew up while he was on-call). No lawsuit for him.
2) Freedom of religion (not freedom FROM religion). He should be free to express his religious beliefs.
This is a toss-up for me. I'm going Kerry on this one (I'm for it, I'm against it, I'm for it). LOL
It might appear that way, but maybe not.
"He asked for, and received, accommodation for his belief that he should not work on Saturday"
If this constituted an employment contract with the company, he probably has grounds to stand on.