He needs only information from 3 tax returns or a viable explanation for why he was not required to file for those years. If he WAS required to file, then he needs to file. If he didn't file because his worldwide income was below the filing threshold, then that's his reason. If not, he needs to file. You as a joint sponsor do not change that requirement.
Chances are that unless his wife is coming from India, he doesn't need a joint sponsor, once he's working at the new US Job. Why isn't HE here asking these questions?