Not that I want anypart of this one but...
Chest size on a man determines the coat size. It is a simple around the chest measurement in inches. I am a 46. I'd say I am about average, not thin not fat but hey I admit I have the over 40 beer thing happening OK. Then you need to know is he a short medium or tall person. If he is under 5'8 I'd say short. Over six feet, I'd say tall, in between, you have it figured out. The coats go by S for short R for regular or L long for tall. Waist size is a no brainer, then you need inseam for pant length. It is that simple.
Good luck,