(((((hugs)))))
It is hard when they are so little. Do you know what about a haircut bothers him? Is he always like this at haircuts? My easy child used to scream her head off during a haircut - she would actually scream until she passed out/fell asleep. It was hideous. My mother used to go on and on about how awful I was for not getting her hair cut. I finally told my mother that if it bothered her that much then SHE was responsible and would take Jess for haircuts. My mother lived 600 or so miles away and only came to town 2x a year. She was horrified wehn J screamed through a haircut and got into a TOTAL power trip with J. I just hid a smile because my mother SWORE it was because we took J to "awful" places and that J would NEVER do that in a "nice" salon. LOLOLOL
Of course it has been 10+ yrs since then so I can laugh, but it was SOOOOOOOO awful at the time.
with my boys it was mroe of a sensory thing. Neither wanted to have their hair combed or otherwise messed with, partly because they have eczema and always had it on their scalp and it was hard to treat. I actually bought clippers at walmart and would just shave them instead of going for haircuts because they fought so hard. Wiz almost gave one poor lady a black eye as she tried to cut his hair. He had a TON of tactile defensiveness and lashed out when anyone touched him lightly. A firm touch was okay, but for some reason a light touch truly felt like he was being punched HARD. It is part of his sensory problems. Then as a teen he went to a vocational school and would have the kids in the beauty school there turn his hair all sorts of colors and do something new about every week. So this will pass at some point usually. For Wiz it was having the girls fuss over him that changed things, lol.
My youngest hates haircuts but also hates his curly hair. they recently cut over 4 inches off and you could barely tell because it just always looks about the same.
If haircuts are not a HUGE priority, and he fights like this over them all the time, you may want to make them very very low on the priority list. He may not look great, but is it worth this fuss? It may be, only you can tell. It wasnt' for us but was for my mother. NO haircut I got for them was ever good enough until I just put her in charge of them. I never really cared about their hair as long as it was clean and their scalp was healthy. With the scalp eczema it often wasn't and combing/brushing could HURT. Often with the boys we had to shave it with clippers to treat the skin otherwise it got infected and THAT was incredibly painful.
I hope you can keep calm and see the humor in a 5yo sitting nekkid in a car screaming and covering himself with rice cake crumbs! Making him buy another bag of rice cakes with his money ANd making him clean up the crumbs is an EXCELLENT logical and natural consequence. If possible, go through a drive through and get a coffee or coke or mocha for yourself and NOTHING for him and enjoy the drink until he is done wtih his tantrum.
(((((hugs)))))