In the article Boys hair: long or short by Christina Sbarro, her husband argues, "...boys with long hair grow up to be irresponsible, carefree, wanderers, who don't know how to hold down a regular job. Said long haired boys might join a low-life garage band, or be influenced by the wrong crowd, and in general, would be disadvantaged in the corporate world of "grown up" America." Do you think she agreed? Nope.
If it were completely my decision, I would let my young boy have whatever hairstyle he wanted. If he started to misbehave, I would grab a big can of "no more video games" before grabbing the clippers.
As an adult he may need to worry about how others perceive him, but as a child I need to worry about how he perceives himself.