how to put someone in their place in a workplace-honouring way?

i'm a 23 year old nonbinary journeyman and the only person trained in my field doing this job, so i do the planning, execution and delegation related to my areas.
i recently got an assistant, who is just a highschooler that insists he's "self made" and has experience in my trade- and is of the belief that because we're close enough in age and i'm a little socially awkward he actually runs the show.

i've been trying a lot of things. getting to know him and chatting about my experience which i am going on 6 years of now, letting him do things his way and course correcting when it goes wrong, throwing him to the wolves and telling him exactly how he fucked up the task and now needs to redo his work and fix it- nothing shakes his stupid confidence. i even turned the sprinklers on him today after multiple warnings he was moving too slow.
lately, i tell him explicitly not to do something- and he will do it anyways right in front of me.

short of just going off on this guy, which i am seriously considering next: what do you even do? when you have the experience, qualifications, job title and authority- and boys still won't budge for you?