Hi A.!
I used to cut my son's hair in the bathtub, which always worked because he was always playing intently and never payed attention to me. Well, once he got older, he FREAKED out when he saw some of his hair fall into the tub. He cried and cried until he looked at himself in the mirror and saw that his head was intact. So, no more in the tub.
I then started to cut it while he would play and watch his favorite show. If he moved, I just followed him around. Sometimes it took 2 hours to get everything done and even. But at least it was done!
One day I was cutting the ends of my hair, and he instantly became interested. He pulled at his own hair and said "help". Now he loves it. He likes watching his hair fall and touching it. I go slow, and at his pace. And I never make him sit against his will. I only cut when he'd in a good mood. He also has a special chair he sits in. It always seems to me that whenever I over explain things to my son and include him in things, the more cooperative he is. So maybe if your son picks out a special hair or a special snack or a special movie, he might be more apt to sitting?
Good luck!