How to teach a dense Taekwondo guy in class

Serting the scene:

Been trainning in Muay Thai and Muay Boran (currently Khan 5) since 2018. Joined this gym arround a year and a half ago, and pretty much the people that train are pretty close and good friends.

About 6- months ago, two new guys joined. Both from Taekwondo, friends to each other, one is about 22, and the other in his thirties. The second one is a black belt in Taekwondo, and according to him, been trainning for 20 years in that sport.

Problems arrise, this guy (name J) is pretty respectful, kind of stoic, amicable (even funnier with compared to his friend, that has a way to big of an ego but doesn’t havy anything to sustain it), and it is nice to train with him. The problem? Sparring.

First, the guy trains and spars with 12 oz gloves. He’s roughly 5’9 185-190 pounds, pretty huge legs, athletic, with really powerful kicks. Been told multiple times to get 16 oz to spar, at the very least 14. He says that he’ll do, still hasn’t.

Second, the guy pulls absolutely no punches. Every time he spars, he hits as hard as he can. Spinning kicks, head kicks, spinning heel kick, hook kicks, spinning liver kicks, every time, full force. No matter the opponent. He could be sparring me, other fighters, newbies, the old guy that trains for health, the woman, he will go all out. Even if you say “light spar, technique”, he will go “ok”, and then spam the heel kick.

Third, he refuses to implement anything from Muay Thai, to his detriment. His punches and guard are on par with Duffy Duck, no check kicks, no clinch.

There has been a lot of instances where he goes all out during sparring, and it’s getting tiresome. Cause you will approach him, tell him “go light, technique, implement what you’ve learned”, and back to square one. Coach has talked to him, but after some time, the coach’s instructions are to only allow him to spar with the fighters, and chewed him every time he goes full force on a newbie or someone weaker than him. And every time before, he comes and quietly says “just go hard with him, to see if he actually learns this time”.

But last week, I got pretty tired of the dinamic. Told him “technical”, lasted 4 seconds until he threw a full force head kick. Then spend the last sparring rounds just abusing his leg with low kicks and knees to the quad, but not a single check or plead to go light. Then, I got mad when he threw and elbow and tried to do a judo throw, so just went pretty hard.

My inquiry is cause, I don’t want to keep the dinamic. Pretty much coach puts him to spar with the fighters cause, well, protection of the rest. And the thing is, if it keeps up, J is gonna get injured, maybe even bad. Cause my good faith is pretty much runned up, cause he can’t go technical and implement Muay Thai if he refuses to go light, and if he keeps not plugging the huge holes in his guard and keeps hitting hard, someone or even me is gonna eventually hurt him or injure him.

So advice on to actually help him, cause it would be so easy to just play his game and just pummel him every sparring, but I wanna help the guy