Nehmah, I have been thinking about this a lot. Coats and shirts and jackets ought to be relatively straightforward, but you are entering an entirely new realm of physics and fluid dynamics when you cross from say, a 40 inch waist to a 60 inch one. I'd start with a pair of the largest sweatpants or PJs you can find and just study a little on the way things hang. I expect his legs are a lot larger than mine, but they are still noticeably smaller than that substantial waist. And that means things that would hang from the waistband must taper a lot, for instance. I would strongly advocate that the gentleman consider wearing braces or suspenders. This forum contains many kilted "gentlemen of substance" and you might PM one of them for more information as to their own experiences. Good luck
M'll