The answer really depends on how old your son is. If he's a toddler, he may be a lot easier to convince than a moody teenager!
My son doesn't want a haircut
I'm sorry to say this, but personally I'd be tempted to whisk him off to the barber, sit him down in the chair, … Read more
Who's the parent here anyway that you have to ask?
If he's 16 or under, take him to the barber and tell the barber how to cut it - end of story.
Now if he's older, you can always use negotiation:
Want to drive the car? Get a haircut.
Want that Guitar Hero III game? … Read more