@ucbalumnus My guys are definitely not off the rack. Every so often I drag them to a store where they can be properly measured. Neck, shoulders, chest, waist… the whole kit and kaboodle. Most stores can’t deal with their neck size and refer us to their on line store. With measurements in hand, I have more options and can shop around. S is 5’8" and 18 so his body will continue to change for a while. H is a lot more settled in his body 
Neither H nor S require/demand expensive shirts. Just ones that let them breath and don’t look like re-purposed tenting.