All you really need to worry about is the ribbed part that goes around the head, right? So the pattern has a gauge of 4.5 st per inch and you cast on about 78 stitches. So that is basically a finished circumference of about 17.3 inches. However, it is done in ribbing, so you definitely have some room to play there. I would measure his head in inches, then multiply the measurement by 4.5 to see the approximate stitches to cast on. All of that is dependent on the gauge given, so if you do a swatch and your gauge is different, just substitute your gauge for the 4.5 number.
I hope this makes sense!