The best way to find the right size board is to look at the manufacturers recommended weight range. This will be printed on the boards themselves, on the manufacturers websites and on most online retailers. They put a lot of effort into designing them that way. A huge myth is that height is the most important factor, its just a minor one.
Weight is the most important factor in deciding snowboard size. I would suggest that when you find a board you want to buy, find the size that recommends your son's weight closest to the middle of the range. you could go for a board where he is on the lower end of the weight range so that he will grow into it.
However just consider that the longer and stiffer a board is, the harder it is to turn on, the less forgiving it is; and therefore the harder it will be to learn on. So don't get anything too big. Dont worry about him outgrowing a board too much, lots of people ride on small boards for their weight.
However make sure boots are the right size. You should cheat on that.
On another note, be careful with your 6yo starting snowboarding at that age. I am an instructor and know that kids at that age have very little limb control. Coordination starts at the core and moves outwards with age. 7 1/2 - 8 yrs is when most kids have enough limb control to learn snowboarding well.
In saying that i do not disencourage you starting your 6yo. Lots of kids start really young. But keep in mind that what he can do will be very limited.