It all depends on how often he will be going. If he was going to go a lot you wouldn't want to buy a beginner board because you would have to get another one to suite his skill level. This is where a short board comes in handy, at that young age it is like a larger board but they will eventually grow into it. I started on a 6'2 at the age of ten and 8 years later I'm still riding a 6'2. It just shows that you can grow into a shape or progress skill wise. Whereas if you were to be going not that often a beginner to intermediate board would suite. So probably i would recommend something about 6'2-6'4 for the little fella.

Give him one of your old boards so you can justify buying a new one for your self

as up ther said, depends how much he is going to go surfing. i suggest maybe a soft board. elmino or somthing boards. that was my 1st board