This is kind of an off the wall though that may not apply but the binder is a close fitting garment and as such, anything that would normally be a minor problem may become a major problem. I always wash anything new because the sizing can sometimes irritate my skin. Excessive soap in the wash resulting in an improper rinse could be a problem and in the case of my roommate, some brands of soap caused skin irritation. Last but not least, binders use elastic which contains rubber like Latex. For some people, Latex rubber can cause a reaction.
One other possibly is if he wears the binder more than 8 hours a day, his skin may not have had time to recover from the last wearing. I understand the desire to bind however a binder isn't a great solution to the problem though it's far better than nothing.