I am SBC, my brother-in-law is pentecostal......we have agreed to disagree on a lot of things. We are both saved, which is the important point.
Are you sure your brother-in-law doesn't think he's saved because he believed? And therefore, his salvation rests on something he has done rather than on what Christ has done?
He most likely believes that he is saved because he was smart enough to recognize the truth of the gospel and then, he believed it.