crf100rider21 wrote: he will want more power soon,
first bike I bought for my boy @ 12 (he's now 14) was the 230f. he was a little taller than your son.
One factor I have not heard mentioned is your judgment of how aggressive your son is by nature. If he is more timid, then he will probably love a smaller bike (e.g. the 100 or 150f). But if he is pretty aggressive in other areas of life (and has some natural athletic ability), he will probably outgrow those bikes relatively quickly, resulting in your needing to off load one and move to a bigger bike.
That was the case for my son. He took riding pretty quickly. It was only one season before he was looking for more power than the 230f could give. We ended up getting a YZf 250. He loves it.
Hope that perspective helps some. Good luck. Riding with your boy will be an absolute blast!