There has grown something of a cult of shaving in the last several years. Shaving soaps of the type that Ivanstein is talking about are generally considered "the best" because of the way they build lather, cushion the blade, and help it slide.
But make no mistake that this is still a matter of personal preference, and shaving with a soap and brush takes longer than just slathering on some shaving lotion, too. Cremo is decidedly better than almost anything that comes in a can, so the question you need answered is "Is he using it because he doesn't know there are better choices, or because he actually prefers that style?" Maybe his morning routine and habits don't allow for the time it takes to "wet shave" (the usual descriptor of the process of soap & brush, straight or double-edged razor shaving). Maybe he specifically likes the speed of the Cremo lotion and a quick swipe with that Dollar Shave Club cartridge to get out the door fast.
So I think you should probably ask him whether he's interested in using a soap and brush at all before you get too deep into this. If he is, I think maybe I'd just buy one until you find out if he likes the process - unless you already have KOH and stearic acid on hand, that is. Something middle-of-the-road in price and quality like Taylor of Old Bond, or Proraso, maybe Wool Fat. Then if he likes it, then you get to go wild with making it for him from then on out.