If he's got another knit hat, measure that. Head sizes vary a lot - my head is larger than my 6'1 son. Here's a 'recipe'
that works with any size head, yarn or needles. You'll have to knit up a sample of about 20 sts to see how many sts per inch you get, then use that times the head size to see how many to cast on. The 16" needles will be fine, you probably won't need more than 20" or so, and dpns are usually used after some decs because the sts won't fit on the needles any more. But you can use a longer needle and the magic loop or single loop method to finish it.