I would say recommend a longer style firstly. Something shorter is harder so will be more stress for you while he is getting more stressed. His parents should know what relaxes him the most, do it then if you can. You have already done another part I would recommend which was show him the tools.
Could it be the itchys and thought of itchys that get him stressed? Ask his parents to give him a good covering with talc first, its what I do for children who get itchy as I was one of the children who would scream the salon down because I didn't want a cut.
Lastly is give up, not completley, but maybe try graders by his mum or dad. I would only let my mums friend cut mine, she wasn't a hairdresser, but the only one I allowed to cut my hair. This isn't giving up on him as much as finding what he's comfortable with. But it will be a long and probably slow process finding a way to cut his hair xoxo