For the exact size, you can use a soft tape to measurement of the head, (with a tape measurement) two fingers above the ears and in the middle of the forehead (one or two fingers, depending on how you like to wear it) where it would normally sit the hat on his head.
o measure your head you can either use a soft tape measure or improvise with string and a ruler. Place it around the circumference of your head, about half a centimetre or a finger's width above your ears, across your mid forehead, and around the widest part of the back of your head. Hold the tape firmly, but not too tight. Basically you need to measure your head exactly where the hat will sit. If your measurement falls between sizes or if you prefer a looser fit, go up to the next whole size.
