Not sure how requesting a business case for a payrise during a pay freeze would be a 'ruse'. A ruse to achieve what?
He should construct a business case outlining why the company need (Note the word 'need' not 'should'. 'Should' isn't enough really particularly in this climate) to pay him more containing the following information:
-Any relevant salary survey information he can gather.
-Details of similar jobs currently available in the same or very similar sectors, in comparable organisations. Ideally these jobs should not require the qualifications he has not got. These will demonstrate that he has a realistic chance of getting the higher salary he is requesting elsewhere and therefore could easily leave if he chose to.
-Details of how well he has performed over the last 15 months, meeting/exceeding all his objectives, the significant achievements he has made and details of the positive impact he has had on the team.
With regard to the issue of his qualifications, there are arguments on both sides. If he has been performing well in the job since he arrived he can easily make the argument that those qualifications are not necessary anyway. However just because qualifications are not strictly necessary doesn't mean the market rate for people who have those qualifications shouldn't be higher than those without. So ideally examples of other jobs should not require them.
I'm not a fan of 'threatening to leave'. It is unlikely to endear him to anyone helpful. It runs the risk of coming across as childish petulance, and showing an inflated sense of his own importance (nobody is irreplaceable), and will sound like an empty threat lacking credibility in this climate where employers may think recruiting isn't going to be too problematic and there is no need to pay high salaries to attract candidates.
Anyone coming to me threatening to leave unless they get more money is likely to be met with this face followed by a polite request to go away and provide decent evidence as to why the requested salary is appropriate, at which point it will seriously be considered. Once they've left my office I would probably be rolling my eyes .
Foot-stamping is unlikely to get relevant people on-side and even if it were to achieve the desired result in the short-term, is unlikely to do him any favours in the long-term.
It may well be the case that the prospect of losing your DH might result in a higher salary being offered, but unless the threat to leave comes accompanied by a resignation letter and details of a job offer at a higher salary, it's unlikely to be taken seriously ime.
I would suggest he goes ahead and produces a credible business case and at the same time starts looking elsewhere. If the business case doesn't do the trick, he will be on the way to being in a position to resign with a job offer which should be his next step.