I haven't a pattern for a sun hat, but they are pretty easy to do. You need a round circle for the crown. Measure your son's head or try one on in kmart to get the size. Then you will need a flat strip of material to go around the crown, and another round strip for the brim. For my daughter, I had a 50cm crown (remember to add seam allowances), 5cm rise, and 5cm brim.
You will also need some interfacing to stiffen the brim.
To put together, iron the interfacing onto the brim. Attach the brim and the rise together. Sew back seam, press out your seam, then top stitch. This is then attached to the crown, which is the hardest step.
I also lined my one, by making the lining the same what, then attaching it to the main hat.
I hope this makes sense. If not, a company called Wild Ginger has a free pattern program, which has a pattern for a sunhat on it, that can be customised to your measurements.