I'm a diabetic and a frequent flyer. Make sure to have insulin in original packing, with perscription label. A note from his doctor wouldn't hurt, but isn't guaranteed to help, either -- it's the label they want to see, as there's some concern about doctor's notes being forged.

Most of the time, I'm not even questioned about the lancets and needles, but there have been annoying exceptions. Generally in my experience, the larger airports that handle lots of traffic don't bat an eyelash (they see this stuff, and know what it is and what it's for), but smaller airports may be problematic (I had a BAD experience at Norfolk where an overzealous screener was taking caps off of sterile needles and insisting I couldn't go through, even when I showed him the mandated documentation. I had to call over a supervisor, who took one look and waved me through)