B.L.
If your son is old enough to care, it should be however long he wants it to be and is able to take care of in a reasonable time. This is one place where my ex and I disagree -- I feel that the way one wears one's hair is a rather harmless form of personal expression, and that children should be able to express their individuality that way if they want to. After all, there are far worse ways of doing that than having your hair be long, or a mohawk, or very very short, or spiked, or purple, or whatever.