Seems fear of hair cuts has a name 'Tonsurephobia' ... so maybe he needs some therapy ?
Ah bless ... he has got himself into a corner, and no persuasion is going to help ... will just have to go along with his stubborness and let it grow for a while. Maybe once you stop giving him the attention, then he may ask of his own accord.
Best of luck Emzy .. my son would never sit on the plank at the barbers for a few years, only on my knee.