I just cut Harrison's hair whilst he was watching "Elmo" on TV.
He was distracted and he loves getting his hair combed so it all worked out in the end.
I agree that distraction is the best thing. Take along a book, or something that he likes to look at or play with whilst he gets his hair cut. Or, try getting a hairdresser to come to you... maybe he will be better in the comfort of his own home rather than a salon? Or even try doing it yourself, if you feel comfortable enough.