There are many angles of attack. I myself used the common law. It was honorable, fact based, and ultimately, successful. But it took months.

For this situation, I would use state and federal statutes against the employer. The easiest is a claim of religious discrimination, because it's extremely hard to disprove. I realized that he already tried a religious exemption; being denied only adds evidence of discrimination. In the U.S. one has the legal right not to be discriminated against for their religion -- possessing an exemption is irrelevant. If nothing else, the fact that Covid vaccines are derived from protein testing using the abortion-derived cell line HEK-293, is ground enough for religious objections.

The thing to do is to file a complaint with the state agency that handles claims of discrimination. They will either launch an investigation, or give you a "right to sue." Not that you need their permission, but this shows the judge that you exhausted all courses of remedy.

At this point he can sue, or file yet another claim with the federal EEOC. Since it will make even more noise for the employer, this is what I would do. Again, they will investigate or say go ahead and sue.

While all this is going on (and it doesn't take that long, since everything can be done on-line), the employer will be forced to allow discovery. For example, they will have to say exactly why he is going to be fired, and why his exemption was denied. It's very hard not to give the game away while being under discovery.

Because he is fighting back by showing clear violations of the law (and not human rights, vaxx dangers, or other things that might be argued), the company will likely want to settle. If nothing else, he will cost the company lots of money in legal fees.