You don't NEED any qualifications to do the job. Unless you feel you want to get them for your own knowledge base don't bother. If you do feel you need to know more then do as little or as much as you wish. Night classes, college courses, etc, though anything especially formal is probably going to need co-operation from your present employers.
The new firm will certainly have professional accountants who will be able to help with anything overly technical on the financial site over and above normal book-keeping and debt collection.
Reading between the lines here (perhaps wrongly) it looks like he thinks you'd be perfect but perhaps the other directors need some convincing and you want to show some degree of financial competence? It can sometimes be an awkward situation where three unrelated people set up in business and the finances are run by the wife / partner of one of them.